Every expansion since Cataclysm has hinted at the expansion AFTER the next.
Look at the final villains.
Mists of Pandaria: Garrosh Hellscream. Began his arc in Cataclysm, and kicked off Warlords.
Warlords of Draenor: Archimonde. Foreshadowed by Wrathion in Mists of Pandaria, kicks off Legion.
Legion: Argus. Foreshadowed by Wrathion in Mists, kicks off... Shadowlands?
Battle for Azeroth: N'zoth: Foreshadowed all the way back in Cataclysm with the Twilight's Hammer.... But also the Sha in Mists and Shadowmoon Valley in Warlords.
Shadowlands: All of this foreshadowing started two expansions ago, in Legion, with Sylvanas becoming Warchief and being a grade-A mustache twirler. And Argus' death knocking the Arbiter out cold.
10.0: Wont be a direct sequel to whatever happens to us in the beginning of Shadowlands.... But from Battle for Azeroth! The sword in Silithus, the Old Gods. Maybe with the Void being so terrified of Sylvanas, the Jailer's entire purpose could be revealed to end the war of Light and Shadow?
Void vs Light expansion confirmed!!!!!!