Given the Fall of Cadia, Biel Tan, et all, it wouldn't surprise me if GW kept some sort of renegade/chaos faction as "the bad guys". It would certainly let them keep the hooks from the more recent supplements to pull new players deeper into the hobby after their starter-set games. In a "If you enjoyed this particular story, why not check out the next chapter in the X-saga".
It's not outside the realms of possibility that they might do multiple, different starter sets narrated around different conflicts. That would be pretty cool to see, actually. It would also help new players start the hobby with an army they really want rather than the recommended poster boys of the particular edition and then GW can hook them with further narrative supplements.
One thing I would like to see from 8th edition is the lethality and the sheer scale of the game just generally toned down a bit. Right now there's players dropping ridiculously huge points values on the table to use gargantuan and unwieldy formations in games that either last multiple days or a seldom actually finished. GW had a system for that, it was called Epic. I'd like to see formations remain a strong part of the game but I'd like to see army builds where you're encouraged to use a variety of different toys from the box rather than one or two units en masse. As it stands the general crux of the game seems to be to max out on the most efficient units from your codex. I get that only "competitive" players really do that to any significant degree, but I don't think it's good that the mechanics practically encourage it. So what happens is units get blasted off the board whole-sale without even rolling a dice beyond a cover save in some cases. That's kinda not fun, if you put a unit on the board, getting at least a turn out of it seems only fair. So in that regard, I hope any new starter sets give you a wide variety of toys to play with.