I think we need an expansion with a clear central villain again. One that is there from beginning to end. We are introduced to him in the beginning, we slowly build up to him, and then the final raid is about taking him down. We've only ever really had an expansion that pulled that off fairly well with Wrath of the Lich King. He might've been out there a bit too much, but the general structure was alright. Probably one of the reasons why it was one of the best expansions.
So I would hope for something like that. Possibly a Northrend revamp with Bolvar as the villain. All zones of Northrend revamped with use of max level world quests plus 5 new zones.