To clarify, the expansion in question gets overwritten with a plot of your choosing to follow up the previous one.
Me personally? I would take Cataclysm and merge it with Battle for Azeroth to create a fully-fledged Great Sea / PvP expansion, complete with some sort of naval travel / combat system that would allow you to explore the entirety of the ocean. Following the events of the Wrathgate, it would feature a renewed conflict between the Alliance and Horde that takes place on the open seas.
The main antagonist of the expansion would be Varian Wrynn for the Horde, and Garrosh Hellscream for the Alliance. Both characters would be developed as protagonists for their respective factions - complete with moments highlighting their positive qualities - while they would be developed as antagonists for the opposing factions - complete with moments highlighting their more negative qualities. Their foils would be Anduin Wrynn and Thrall (who is still Warchief) respectively, who would both be urging the other down from the faction conflict. The "war campaign" of the expansion would essentially consist of two options: a pro-war approach - led by Varian / Garrosh - and an anti-war approach - led by Anduin / Thrall.
The final raid of the expansion would culminate in a naval assault on Stormwind City that mirrors the events of the First War, with a defending Varian as the final boss for Horde players and an invading Garrosh as the final boss for Alliance players.