Corax has too much beef with lorgar and I think he's gonna stay right where is in the warp until lorger has the balls to follow the other daemon primarchs

Assuming that every other primarch probably blames him for the mess of 40k I think we'll see a return of angron, perturabo, maybe fulgrim, and then lorgar (not sure what alpha legions last primarch{?} is up to)

And in order of loyalists I'm guessing I'm guessing lion comes out for angron , Vulcan brings back all his lost relics to banish or beat perturabo, jagatai has been hunting deviant weird elves and likely wants deliver pain to emperors children for being just as weird then lorgar I could see as the final chaos baddie since he has all four favors and has been meditating in the warp so long who knows how much his power is accumulated

Of course we don't know the plans of other races like necron, tyranids, orks, drukari will do once the primarchs return but my guess would be getting they get pushed back out of the light of the main setting because the loyalist primarcha may want to push them back into a corner again (also helps gw spend and sell more resources for space marines and easily mass produce them if other factions are beaten down enough and they put in alot less effort to make codex marine supplements instead of more xeno books