With the info we have at the moment, the only "reasonable" expectation is void/light/death stuff. Wow does have a track record of unexpected twists though... I'd have expected void/light to be BfA after all.
I hope for a new "pandaria-like" expansion, where we get to explore new unknown "filler" lands with tons of new lore before unavoidably having everything be somehow connected to one of the primordial forces that have complete control over wow plots