Here's my crazy (maybe not crazy) take on 9.0 and beyond, cobbled together from my own thoughts and some ideas I've seen on these forums:
At the end of BfA, we lose. Alliance AND Horde lose.
N'zoth starts doing its thing, presumably waking up other old gods and working on the return of the black empire. Azeroth presumably doesn't get corrupted so players still have one victory to hold on to at the end of the expansion.
Pre-9.0 introduces a need for us to go to the shadowlands to seek help. Maybe because of some crazy scheme to get back fallen heroes; maybe a bunch of big names die at the end of BfA. One way or the other, there's a need to find a way to ally ourselves with the forces of death/undeath to defeat the old gods.
9.0 launches with primary content in the shadowlands. "Old world"/Azeroth is greatly diminished due to N'zoth's efforts, to the point that characters have less power than before (read: level squish). The Alliance and Horde finally stop fighting each other, too devastated from the conclusion of BfA to afford more war. The factions dissolve. Instead, players align their characters with a faction of Life or Death; both with the ultimate goal of defeating the old gods, but taking different approaches and following different philosophies. Rather than getting Garrosh'ed in BfA, Sylvanas becomes the defacto leader (or one of several leaders) of this Death faction.
9.0 does not introduce a new class, it introduces new specs to existing classes. A DK spec that's more of a caster like back in WC2. Dark Ranger spec for hunter. Etc.
10.0 is when we take the fight to N'zoth. Maybe the Dragon Isles gets involved here. Maybe Azeroth gets corrupted and set up for the 11.0 BBEG.