Advancing the Storyline
Advancing the Storyline is what a great number of neckbeards believe that Games Workshop need to do with Warhammer 40000. On /tg/, Warseer, Bolter and Chainsword, and Dakka Dakka, people complain and grumble about how the storyline never moves beyond the year 999.M41, with Abaddon the Despoiler's 13th Black Crusade on the very brink of taking Cadia, for real this time, the Tyranid Hive Fleets closing in on Terra, the Astronomican flickering and fading, and the Golden Throne one Adeptus Custodes's sneeze from shutting down permanently.
Why they're wrong
Some people - Aaron Dembski-Bowden being one of its most frequent proponents - hold the view that this attitude is a load of shit, and that it completely misunderstands the nature of the 40k setting.
This is because 40k isn't a story, and in fact, doesn't have a "storyline"; while events from it, such as the Black Crusades and the Badab War have had their stories told, there's no one story that the setting exists to tell (unlike with universes such as those of Star Wars or Doctor Who; even though other stories exist in those settings, they're based on a single one). 40k is a setting in which stories take place, and has 10 thousand years and a whole galaxy in which to set them, so expecting the timeline to "advance" to "continue" or "finish" the "story" is a stupid idea.
The other, more practical issue with advancing the storyline that a major change to the storyline is likely to have severe reprecussions on one or more of the different factions, which may not always be welcome changes. To use the most obvious example, consider what effect story progression would have on the Imperium of Man. The majority of 40k players favor one of the many Imperium-aligned factions-assuming that the God-Emperor doesn't get resurrected and the Golden Throne isn't fixed before it fails (which itself is rather unlikely and has a good chance of causing problems of its own), the sheer number of threats that the Imperium faces on a constant basis will tear it apart as soon as the Emperor snuffs it, leading to the enslavement and/or destruction of humanity. No Imperium means that about half of the armies currently in the game will no longer exist, leaving numerous fa/tg/uys stuck with unusable armies and a serious grudge over being given the Squat treatment. Needless to say, Games Workshop's profits would be hit incredibly hard by the departure of so many paying customers, so they have no choice but to keep the Imperium afloat. Although this has the infuriating side-effect of causing the setting to grow stagnant and unchanging (much like the Imperium itself), GW can't afford to appease one group of complaining neckbeards over another which would complain even more loudly if their armies were suddenly made unusable.
At the same time, it would be equally risky for GW to risk upsetting the status quo for any other race- for example, if the Tyranids started arriving in full force, the Tau would cease to exist as well because they'd be the first to get nommed. Naturally, this would infuriate Tau players, and the Eldar dying out completely and forming Ynnead would meet from an equally chilly reception from both Eldar and Slaanesh players. Similarly, reunited Necrons would be such a juggernaut that they'd be able to wipe out all other factions effortlessly, which is also something GW wants to avoid. If the removal of the Squats (which were always a rather small army with only a handful of players) was enough to produce a major outcry, then the rage produced by the removal of a major faction will be truly unimaginable.
Why they're right
On the OTHER hand, Privateer Press has managed to pull off a metaplot in a wargame just fine with WARMACHINE and Hordes, likewise Battletech, and there's no reason that it should be any different for 40k. Besides, given the fact that GW is already expanding the scope of the game to include the previously untouchable events of the Horus Heresy, it's perfectly possible for them (and probably quite profitable since it would give them an excuse to make a new line of minis) to start encompassing events further into the future as well as into the past of the setting. (Some can say that they're already doing so now with the increased emphasis on the "Time of Ending" in the current Codexes.)
On top of that, it can be argued that the central story of 40k is the story of the Imperium's fall from glory and slow decline, which must by definition end with either the Emperor getting revived or the destruction of the Imperium of Man, and failing to resolve this central storyline is slowly causing the whole story to stagnate as it runs out of events and gaps to fill in. Even the evolving stories that your dudes could once be capable of can no longer exist because there is simply nothing left to evolve. Remember how the Eye of Terror Campaign ended in a victory for Chaos? Instead of allowing its results to change the background (via Abbadon taking Cadia), GW instead decided to backpedal in a way that ultimately made the events of the campaign utterly meaningless. How can you have an emergent narrative take place when any sign that it might upset the way things are now results in it being retconned or otherwise made insignificant?
Another major problem caused by the setting's stagnation is the presence of numerous plotholes which form as a byproduct of GW's insistence in squeezing the shit out of 999M41- a good example of this is the Knights of Blood to defend Baal AND also to attack the Farsight Enclaves on the same year despite the fact that they should be on the opposite sides of the galaxy from one another. The only way to fix that would be to retcon the date, which would create problems of its own depending on where they inserted the new date.
The biggest problem is that Games Workshop loves their status quo. They'll advance the story in bits and pieces but never anything that changes the status quo. In 40K, in Games Workshop's vision;
- The Imperium will always be stagnant and rotting, but they'll never be destroyed or fractured (helps that they're a Creator's Pet and therefore the best selling faction).
- The Orks will always just wage wars for fun, but never unite to pose a threat to the galaxy (surprisingly, given Ork nature, this makes sense).
- The Eldar will always be full or arrogant people, dying and trying to rebuild their empire, but never progress, succeed or go extinct.
- The Tau will always be a new, expanding empire with hints of Grimdark beneath their benevolent façade, but never get too Grimdark or expand to the point where they threaten the Imperium.
- The Chaos Space Marines will always be trying to overthrow the Imperium, have a grudge against it and be under Abbadon's leadership, but never succeed in a way that puts the Imperium in jeporady or puts someone besides Abbadon in charge.
- The Chaos Daemons will always be corrupting things and fighting, but never win a lasting victory or suffer a permanent setback.
- The Necrons will always be an ancient empire slowly reawakening with each faction following the dictates of their Overlord, with the C'tan either enslaved or in hiding planning to restore themselves to their former might; but never fully awaken, fully be destroyed, fully unite and the C'tan will never be completely enslaved to the Necrons or completely free.
- The Tyranids will always be a major galactic threat answerable to the Hive Mind and not ally with non-Tyranids, but never win, be wiped out or wipe out or weaken a playable faction.
- The non-playable factions will always get a token mention, but never get time in the limelight or become powerful enough to challenge a major faction.
Rant
- Space travel is still intolerably slow and hazardous for the most aggressive and numerous races. Look at how long it took the Tyranids to even reach the galaxy and how long Szarekh, the Silent King, spent out there until he ran into them and turned around, moving at MORE OR LESS THE SAME SPEED.
- The galaxy is huge. It has 300 billion stars. Accounting for probability and terraforming and alternate needs, a lot of those are going to be inhabitable to somebody or something. Then there are also undeniably 300 billion star systems worth of resources not accounting for starless debris. So put together, you have massive, massive civilizations, that can't eliminate each other at the same speed or even try to unawares, because the intelligence, military and administrative networks are huge, and the supply demands are gastronomic. Consider this: there isn't enough chemical fuel on Earth to get us to Alpha Centauri. Think about how much fuel, food, and whatnot you need for a year campaign with all those damn Leman Russes, Imperator Titans, and millions of Guardsmen and whatever diseases, injuries and damage is incurred, not to mention what this does to the tax rate and the the several million people you're extracting from an economic system for war alongside all the other armies that are fielded. I mean come on. Even Space Marines have this problem and more so because they're equipment is way more expsenive. Hence why Chaos Space Marines cannot get their shit together, because half of their worlds the cultists fight over it for food since their leaders either enjoy watching those fights out because of the bloodshed, the rampant passage of disease through eaten poop, exploring the myriad possibilities and quantum probability of such quarrels amounting into something that they don't understand, or it turns them on, or any other myriad of other reasons. This the essential problem Tyranids have-the Hive Mind or Hive Minds don't create lasting infastructure because they don't care for ecological/economic balance, only for consuming everything to consume more, hence, exploitation negative effects. They are running from their own imminent starvation, whatever happens. Calculated statistics of the Hive Mind may dictate that taking an almost biologically worthless space rock populated only by superhuman warrior monks whose k:d rate is an expensive risk for Tyranids to take may explain why have the Segmentum Ultima isn't being eaten by Tyranid scouting parties that are the equivalent of an Imperial Fleet. They can't take everything or they will lose too much.
- Also, everything in the board game is set in 999M41 BECAUSE IT IS A TURNING POINT. All evidence points from the entire fluff of Games Workshop that logically, unless Tzeentch's ultimate just as planned is to join up with Nurgle and put as much effort as possible into keeping the world the way it is, one of the factions is going to have a severe problem. This is supposed to be slightly irritating, causing thought, predictions, threads, rage or not and creating interest in the future of 40k. It is part of what many of us double-monkeighs experience, anxiety about the future, even in suspending disbelief. So GW is actually trying to give us as much freedom with our imagination in the world they created, and is doing so by picking a point in time when the Imperium's true face is becoming clear to many, a time when the mortal followers of Chaos faces a fanatical danger to their Warp patrons existence due the birth of an equally fanatical cult, the Imperial Cult, when Horus struck down the Emprah, and his semi-Enlightenment expansionist aetheist ubermensch bureaucracy turned into a stagnant crusading religious doctrine madhouse, that demands removal under the pain of death from both sides, when the Eldar face a schism between isolationism and total war, between legalism and liberty, a time when the Space Marines, Guard, Sisters, and Inquisition more than ever find themselves at odds with each other and even more so with the Imperial Senate and the Navigator Houses, a time when the Orks are stuck between their barbaric tradition and becoming more humanistic (the War Axes trade with Imperial neighbors via black market and have adopted many Imperial Guard tactics along with currency, and then there's the Grot Revolutionary Council). The Hive Mind face starvation now or later, and whether to create more independent commanders (Hive Tyrants) to improve elimination of enemies or maintain maximum control of the swarm, and the Tau are stuck in a blatant civil war between the idealistic liberal Farsight Enclaves and the legalist caste system of the Tau Empire, and even the Necrons are stuck between unity against biologics or fighting for power. The pros and cons of picking between division and unity are the ultimate themes of 40k, the lesson being, and the warning, that to succeed in anything there is a price to be paid, whether it be for good or evil, whether it be for law or chaos, there is always something that must be given up in order to succeed. The ultimate debate is what, and each faction has many similar values but are differentiated by what price they choose to pay and what they tell their denizens and rivals what they have to pay and the ultimate reactive effects of doing so. 40K has so much to it. Putting an end to it would leave one conclusion, one possible ending. If you want a thesis on that, go read the Dune books 1-8 or philosophical texts. The problem is, we ourselves have not reached an end to this conflict. So not only is there a transfer from suspense of disbelief to past tense reference archiving in your hippocampus, the same thing that happens when you finish a book or tv series, a definite end to the Warhammer 40k universe would not only be a cultural loss of a beautiful story that references so much of our own history and art provides ideas about where we are going, but GW would lose so much money. While the largest minifig purchases are for the Imperial aligned models , be they the Emperor's Finest or the Emperor's Chosen or the Emperor's Brides, it is the reason they fight, and how they fight, and what they fight, and they fight that makes them interesting, like all the factions. To end this dynamic that holds so much of what it means to be human, the struggle of for individuality, a dedicated art (For war is an art, however grisly, and however driven by other reasons, because it is a skill), the struggle to live, choosing between whether to help others or serve them or to provide only for yourself and why, and the emotions carried, sadness, anger, hatred, happiness, sorrow, rage, depression, jealousy, love, joy, happiness and hope, in tandem with interpreted fact that led to things like the Horus Heresy, or inspired an anonymous write to write True Love Can Bloom, or the trials of the Gabriel Angelos, is what makes 40k of interest, even if, no, especially that manifests in putting two armies against each other with bullshit government pretext and resulting in incredible loss of individual life for a goal that might not even help the combatants in any way, or that is paramount to their survival. You don't have to think this to appreciate it, you don't have to be artsy, analytical or a complete book worm to enjoy 40k. It is simply the fact that despite all the complaints, all the criticism, with basis or not, and all the misperceptions GW may have had of their audience and game mechanic mistakes they may have made, part of the natural relationship between creator and receiver, everyone who plays something from 40k does so because there is some reason they are interested in it, and to me, that is because GW did a veritably good job all round in creating 40k.
Other stuff
There is also the matter that some of the Ciaphas Cain books take place in the early years of M42 (though his adventures are not exactly Imperium-shaking events)- if those can be considered part of the fluff now, what's to stop it from going further than that?
It also bears worth mentioning that changes can be made to the storyline without altering the tabletop. Warhammer Fantasy kills off major characters (for example all the named characters currently available to the Vampire Counts army, and half of the Orcs and Goblins characters) and they are still fieldable in the game. The plot of the setting progresses beyond that point and introduces new characters, encouraging players to not simply play "in the present" but instead just pick someplace in the timeline for their battle. Sure you run into inconsistencies when someone long dead is fighting the army of someone not even born when they were alive. But hey! Necromancy, gods intervening, and Chaos fuckery make a good explanation, as does the age-old rationalization of "shut up and just play the game". If one were to take that approach to 40k via advanced technology of some kind, Warp-related time distortions, or the aforementioned Chaos fuckery, then anyone can appear at any time if the players wish it despite them being killed off in canon. Plot can progress, everyone gets to keep their favorite canon from the past, everyone wins. In fact, this has already happened in canon- Captain Tycho has been dead since the Third War for Armageddon, as is Lord Solar Macharius, but that doesn't stop either of them from being playable. Hell, even Eldrad was dead for a while before the retcon hit.
TL;DR- while the setting does need to be shaken up some, making significant changes to the setting has just as good of a chance of making things worse instead of better.
Beyond the 41st Millennium
Of course, while Games Workshop may never enter the 42nd Millennium, that doesn't stop us from writing up fanfics that do so (or from bickering over which possible portrayal is more likely to actually occur).
- The ship moves, a setting where, in the grim darkness of the 51st Millennium, the God-Emperor of Mankind orders the construction of a giant ark to leave the failing Imperium behind.
- Story:The Shape Of The Nightmare To Come 50k, a plot that manages to become even MORE grimdark than it already was, with the Emprah croaking, the Imperium splintered into Khaine-knows-how-many pieces, and several other incredibly crappy things changing the universe even further.
- Return of the Primarchs, where the fall of Cadia coincides with the fleets of the fallen/dead Primarchs from before the HH, the Lost Primarchs get found and they all band together to help the living ones get up and bring the Imperium to a more presentable state.
Warhammer Fantasy
Unlike Warhammer 40k, the plotline of Warhammer Fantasy does advance, but in small increments. Each edition and army book usually adds a smattering more fluff in the past (and maybe a retcon or two), rarely an update to the big prophesied battle between good and evil that decides the settings future, and a plot hook in the present. For example, the 8th edition Vampire Counts and High Elves army books (Codex for 40k players) added a new story to the end of the army timelines that mentions how Mannfred von Carstein kidnapped the Everqueen's daughter Aliathra, and is going to sacrifice her like a Frazetta painting to bring back the settings big BIG bad Nagash and that the greatest hero of the High Elves, Tyrion, has saved her and is riding at the head of a large High Elf army about to clash with a large Undead army.
Smaller updates (mainly gimmicks to sell a book and some models) like Storm of Magic will add a whole new event that extends the "present day" by a few months to a year. The infamous Warhammer Online was entirely non-canon which may have been what doomed it from the start. Regardless, Fantasy isn't THAT adventurous about advancing this plotline either.