I think that shadespire was a mix of new and fan-favorite factions, and Nightvault à mix of new AoS and upcoming factions.
At this point it is 110% spéculation, but concerning Tzeentch, I could see demonic terrain release along the rumoured Kill team Inquisition expansion.
Concerning the Malign portents follow-up, do you think à campaign book could be released along with the upcoming generic endless spells ?