Well, I’ve been thinking ALOT about this. 
So lets start with the first two expansion books;
Invasion:
- Xirkalli
- Salamanders
- Halflings
- Trident Realms
Fracture:
- Nature
- Basileans
- Forces of the Abyss
- Ogres
With these first two books we can start to see a pattern as to how Mantic’s dividing the armies into each book. The first is the “feature” army - the one that gets a major revision with lots of new kits (ie. Xirkalli and Nature).
The next ones are armies that best fit the narrative they’re telling. So for Invasion it was Salamanders and Trident Realms (two of the main naval factions to oppose the Xirkalli’s own naval invasion) and in Fracture it’s Forces of the Abyss and Basileans (both of whom relate to the Green Lady’s aim of merging each half of the Shining and Wicked Ones).
Finally, there’s the filler army - the ones that don’t need many (any?) new kits for their army list and are not strongly linked to the narrative told in the book. Halflings gave Mantic a chance to expand on the Ej colony in the way of the Xirkalli invasion and Ogres have their homeland directly north of the Abyss. But both armies could have easily been justified in later books too.
So in short, each book should have a main army who’s getting the major revision, one or more armies that are their natural antagonist/ally in the current lore and if needed, a filler army to round it out.
Based on that, here’s what I’m thinking Mantic will do for the future books;
Arrogance:
This is a difficult one to figure out, more-so than any of the other books. As best as I can tell the Elves don’t have an obvious antagonist in the current lore, aside from the Xirkalli. I initially thought Twilight Kin but when it comes to their cousins, Twi-kin seem more interested in spying and subterfuge than outright war.
My best (admittedly poor) guess is that Mantic will make this a classic “Elf vs Greenskin” book. The title is Arrogance, so it seems appropriate that the haughty elves are brought low by the crudest of races.
I initially put Kingdoms of Men in the 4th slot, but I’m sure that’d be wrong. Surely it’d need to be a massive release with heaps of kits? That doesn’t work well with the Elves own big revision. But from a narrative standpoint I can’t think of a better spot for KoM - I can easily imagine them being the force that comes to the Elves aid in their fight against the Greenskins.
But with KoM excluded, I’m thinking this’ll be the first 3-army book this edition.
Ruination:
- Dwarfs
- Free Dwarfs
- Abyssal Dwarfs
- Ratkin
It’s possible that Dwarfs will sit in the “Alliance” book to reflect the Dwarfs and Free Dwarfs finally working together after their civil war, but I feel like Ruination is a better fit. Free Dwarfs aren’t currently an army in their own right, but Mantic has pretty much confirmed that they’re coming back so if they’re going in any book, it’ll be with the Dwarfs.
Narratively the Abyssal Dwarfs have had an active (if subtle) hand in the Dwarf civil war. There’s also clues in the Invasion book that they’re planning a fresh invasion of the Dwarvern lands once they’re at their weakest after the civil war. Ratkin would be the “filler” army for this book, since they occupy parts of the Halpi mountains and probably want to keep their Holds. It could just as easily be goblins for the same reason. But with Ratkin there might be a chance for some model overlap with the Abyssal Dwarf list, depending on how deeply they go with the Ratkin Slave theme.
Alliance:
- Northern Alliance
- Varangur
- Nightstalkers
- Kingdoms of Men
Putting Northern Alliance in the “Alliance” books might seem a bit on the nose, but so does putting Elves in the “Arrogance” book so I think it tracks.
We know Varangur are coming as their own army, so it makes sense to put them alongside the Northern Alliance, both narratively and because there could be some overlap in model kits. Twilight Kin have been gunning for Chill for a while now, but I think they’re better placed in Death (see below). So instead I think the Nightstalkers will be the baddies here, likely acting under some influence of the Twi-kin.
Kingdoms of Men are in the 4th slot because this is the best space for them to get a big revision. They make sense in the broader feel of an “Alliance” book, but otherwise I can’t really be sure how they’d fit into the lore.
Death:
- Undead
- Empire of Dust
- Twilight Kin
- ?
The only reason Undead and Empire of Dust are here is for the title of the book! Honestly though, I think this book will be the set-up for an undead resurgence, leading to the next edition of Kings of War and the release of Ophidians. It’s hard to say exactly which armies will also feature in this book. My best guess is Twilight Kin, who in the current lore have expressed an interest in stealing magical artifacts from the Opidians that they themselves are stealing from the Empire of Dust.
Beyond that, there’s room for a new army (maybe splitting the undead into Necromancers and Vampires/Tetsumo Shogunate? I can dream. ). Otherwise I think this could otherwise be another 3-army book. That in itself would make sense - leave a slot open at the end if an army needs to be pushed back, and it otherwise gives Mantic the design space to prepare for a new edition launch.
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