Grand Alliance: Order
Aligned as one, the civilised races of the Realms go forth to claim the lands and restore Order, Sigmar's Sigmarines Stormcast Eternals leading the fray.
Armies of the Aelves
Look! It's the Elves Aelves! As fancy and effeminate as ever, and now split into 11 12 (!) different factions.
Darkling Covens - Aelven sorceresses (and occasional sorcerer) that didn't agree with the rules of the Eldritch Council and went to do their own thing. They have enthralled whole armies of mostly aelven followers which they use to push their coven's agendas. They have male sorcerers now.
Daughters of Khaine - The followers of Morathi who worship Khaine. As crazy as ever. They're part sadistic Witch Aelf, part crazy Snake Ladies.
Eldritch Council - Groups of sorcerers and their followers that have established enclaves throughout the Free Cities. Still some of the best magic users around.
Lion Rangers - Warrior mystics that roam the land with their lion companions offering aid to those they can. Currently fighting alongside the armies of Order.
Order Draconis - A proud knight order, few fled to Azyr during the Age of Chaos because they held their ground and defended their holds like men and survived.
Order Serpentis - The shadow of the Order Draconis, they're merciless but still proud. They had whole dragon units, but because there are few left they (re-)created the War Hydras and Drakespawn with the help of the Darkling Covens to fill the gap. They have a thing going with the Covens and help each other from time to time.
Phoenix Temple - A warrior order that worships the Ur-Phoenix (which is implied to be a God-beast in the Mortal Realms). Their warriors appear to have sworn oaths of silence and are inducted into the order after almost dying through ice and fire before being revived by a phoenix of the temple.
Scourge Privateers - Apparently now monster hunters. They go around capturing fantastical beasts and selling them (either whole to be tamed and trained or in bits and pieces) to different organizations for some coin.
Shadowblades - AoS' Night Lords. They usually stalk the shadows of the cities of Order watching for who be in cahoots with Chaos or Nagash and gruesomely murder them and put their bodies (or what's left of them) on public display as a warning. But they're good guys.
Swifthawk Agents - The messengers and scouts for the armies of Order. They also maintained waytowers throughout the Realms that still stand today and serve as bastions.
Wanderers - What's left of the Wood Elves. Apparently, the trees don't like them so much anymore ever since they got on Alarielle's bad side.
Idoneth Deepkin - Literally sea elves reborn! Teclis' botched attempt to recreate the elves; their souls are atrophied, so they need to take the souls of order beings to keep themselves alive.
Armies of the Duardin
The Dwarfs Duardin are back, and are as grumpy and beardy as ever.
Dispossessed - Classic, "proper" dorfs. Still holding grudges and generally being grumpy. Some of their heavier units have migrated into the Ironweld Arsenal list.
Fyreslayers - Expanding on the Slayers, here's a whole faction of them (and no, they don't slay fire, incorrectly spelled or otherwise). Ride atop giant salamanders and possess very little in the way of clothing. Obsessed with Ur-Gold, which is said to be composed of Grimnir's remains. For old Slayers, there's the Unforged.
Kharadron Overlords - Steampunk dorfs that lead a meritocratic society. They have airships, guns, blimp-jetpacks, and they... okay, they're Squats, guys. They're basically steampunk Squats with an interest in technology and profit.
Armies of Man
Now relegated to "silly normals" status, mankind are now the Imperial Guard to the Stormcast's Space Marines.
Collegiate Arcane - Assorted wizards and magical scenery. You can now play a full army of dress-wearing spellslingers! Rejoice!
Devoted of Sigmar- A little bit out of place in this new, Noblebright world inhabited by giant immortal armoured warriors who eat Chaos and shit out redemption. The Devoted are the classic raggedy zealots led by warrior priests and witch hunters. Their job is to go in after the Stormcast have taken a new area and cleanse it of Chaotic taint and other malign influences before the new settlers show up, and then keep an eye out for corruption afterward. What, those other names engraved on the War Altar next to Sigmar's? I don't know who you're talking about. What's an Ulric? Also one of the model lines updated indirectly by AoS in the form of the Excelsior Warpriest.
Free Peoples - What remains of the Empire list. It turns out that thousands of years later, in a totally different place in time and space, men and women still dress in exactly the same silly feathered hats and pantaloons. The Free Peoples are the various poeple that fled to Azyr in the Age of Chaos and the original inhabitants of Azyr who are now recolonizing the realms. Since the freeguilds haven't really been updated yet the models still have Empire heraldry and symbols up the wazoo, meaning that Freeguild General that has "Karl Franz" emblazoned across his griffon's armour? Total coincidence... and while we're at it, what the fuck's a Reikland?
Seraphon
The lizards are back, but they are now demon space lizards, apparently. Very blue, very angry at Chaos, and with a new plan to stop the Dark Gods for good, they appear from nowhere to mulch the gribblies at every opportunity. Due to some kind of racial memory, the Skaven are absolutely terrified of them. The Bonesplitterz are just pissed off that their monsters disappear before they can eat them.
Stormcast Eternals
Games Workshop's new golden boys. Space Marines weep, for in their heart they know they will never be a Stormcast Eternal. They are lead by the Lord-Celestant, their Spiritual Liege, who takes Ghal-Maraz out for a spin like he's borrowing the keys to his dad's Ford Fiesta. Notably, he hasn't been named as of yet, so rumour abound as to who he might be.
Stormcast Eternals - It's the Sigmarines! The new poster boys of the setting, and likely the result of several GW board meetings trying to tap into the "male power fantasy" - but then the Sacrosanct Chamber came out with many female 'Casts, so maybe just "power fantasy" - either way, seemed as though it worked. They are essentially the Fantasy answer to 40k's Space Marines, being nigh-immortal demigods who are viewed with mixed awe and fear by both allies and enemies. Unlike Space Marines (barring one or two examples), they basically can't die under normal circumstance. Upon death, they are warped back to Sigmar's side, smacked about with a hammer reforged, then sent back. Nagash doesn't like this, and takes whatever bits of their soul he can grab on their way out, making them come back more and more like automatons. And now that he figured out how to claim whole soul...
Extremis Chamber - It's the Sigmarines, but to the extreme! Totally radical! No, it's The Stormcast Eternals, but on Dracoths and Stardrakes. This faction was later folded into the Stormcast Eternals book proper after the Battletome update, but in-universe is regarded as a somewhat seperate warrior brotherhood who specializes in communing with the many drakes that live in Azyr.
Vanguard Chamber - It's the Eternal's scout regiment; but they're not the snot-nosed greenhorns of the Eternals. No siree, this is the Catachan type of scout regiment (so... reivers?), the type that rips their enemies open and perferates them with automatic crossbow fire. Particularly know for being the Chamber that Neave Blacktalon comes from, a golden demi-human blender with two axes that'd make an Eversor blush.
Sacrosanct Chamber - The Eternals that put on a robe and wizard hat instead of picking up lightning hammers. No scratch that, they do that too, only in badass robes. They were originally a civilian Chamber tasked with overseeing the machinery of the Stormcast Chambers, as well as the reforging process that allows them to return to life. With skelepope rising from his cozy grave alongside about several billion angry, spoopy ghosts, Sigmar determined that maybe the soul-forgers would be good at putting souls to rest as well.
Sylvaneth
The Tree People from the Wood Elves, who they now hate due to momma Alarielle saying that they shouldn't play with those nasty meatbags. Mainly found in Ghyran, but appear in all the Realms.
Others
Ironweld Arsenal - A bunch of Human and Duardin engineers came together and compared notes. Cannons and general siege machines. Seems to be intended for use as an ally for artillery support, but does also have things like gyrocopters included too.
Bretonnia (Warscrolls Compendium: Bretonnia[1]) - Not a faction in the fluff, as its models are now discontinued and they've gone the way of the dodo. Still have rules on the GW website, meaning you can use them to have your own flavours of knightly orders and stinking peasantry.
Pantheon
Sigmar - The God King of mankind, and creator of the Stormcast Eternals. He is spearheading the crusade to take back the realms from the grip of the baddies.
Grugni - Crafts weapons and armour for the Stormcast Eternals as repayment for his debt to Sigmar.
Grimnir - Blown up into Ur-Gold pieces fighting Vulcatrix, is currently being very slowly reassembled by his followers the Fyreslayers.
Alarielle - A bit unhinged after Nurgle's incessant attacks on Ghyran. Was slain in one of her wars against Nurgle, but her soulpod survived to let her come back riding a Giant Beetle and ready to kick Nurgle's fat ass.
Tyrion - He's blind now, can only see through Teclis' eyes. Rules the realm of Hysh.
Teclis - The Co-Ruler of Hysh alongside Tyrion. Had taught most Order Wizards the basics of magic during the Age of Myth. Accidentally created the Idoneth Deepkin.
Malerion- He's a dragonaelfgod now, is still selfserving.
Khaine - At least that's what the Daughters of Khaine believe.
Morathi - Who the Daughters of Khaine are really worshipping unknowingly.
Ur-Phoenix - By the Phoenix Temple Aelves.
Others like Valaya, Kurnoth and Mathlann still get patronage, though it is unknown if they still live.