...Anyway, the army books are mainly driven by game design, which is driven by the idea of creating a couple of key playstyles for each army. As is visible in every level of the T9A project, the fluff is secondary, and may need to be rewritten at times to allow for an emergent playstyle.
That was true for the first edition - and most of current AB are carried over from that time. The idea was to focus first on collection legacy and tournament balance.
The focus has changed for the second edition.
Each army will get progressively a detailed T9A background, which will serve, along with playstyle and model legacy, to define the 2nd edition AB.
Warriors were given full background attention. Daemons as well, plus more freedom to the concept team.
A new organization is being implemented, which will provide even more freedom of creativity to the team in charge of Infernal Dwarves, followed closely by Dread Elves.
Be sure we're moving from the idea of fluff, accessory to playstyle, into the idea of background, source of the faction's soul.
...I'm sure the designers will have a closer look on how the Vermin play and how to make their intended playstyles fun and rewarding. The writing, on the other hand, will remain as obscure and potentially misinformed as in the "legendary" army books. For me personally, that makes the setting totally uninspiring, but of course it leaves lots of room for interpretation and fanfic.
Undying Dynasties and Sylvan Elves will be redone like all others. We have extended since sources of information to be less obscure, albeit still in-world and prone to misinformation from outsiders - especially true for Daemons, for some reason... See the "Scrolls" where many pieces of background have been provided.
...At one point I remember the T9A team wanted to create a dynamic setting, where the present would not be the like of eternal status quo of the WH Old World (where each chaos incursion was battled to a halt, and the wretched and tainted lands returned to their former nature in a generation or so). In this dynamic setting, some fantasy races could be a little downtoned now, but would then emerge as new dominant factions in the next update. IIRC, the Vermin were such a faction - long in the hiding, but ready to embark on a big campaign to assume a leading role in the world politics. Not that different from the WHFB idea. I don't know if this design principle is still held by the writers. But the idea of a slowly changing world is a good one, I think.
This idea is still there.
However, I would not expect a dramatic change which would be the coming of a 10th Age, but rather an evolution of History.
At the moment, High Elves rule the seas (like Victorian UK) and dominate world trade.
The Empire of Sonnstahl is the growing challenger, a land power becoming imperialistic (like Wilhelm 2 Germany).
Other factions regret their former hegemony (like post-Napoleonic France), resist change or anticipate opportunities.
Which big change could occur to alter significantly History has not been thought yet.
I would not expect it to be a Daemon invasion, that would be too close to GW. I would rather anticipate a more modest change, which would be illustrated with a campaign.