I've been wondering this for a while.
It's pretty clear that we're running out of land on Azeroth to stage new expansions. At the same time, it's clear Blizzard has no intention of "ending" the game any time soon. So, where do you think we could go next? We just got back from outer space, we've been to alternate timelines, explored the farthest corners of Azeroth, we went to outer space another time (Outland), so what's next?
Personally,I wouldn't mind revisiting some old locations, though I doubt the fanbase would approve of recycling old zones. I've always hated how WoW's storytelling is constrained by linear progression. Once we "finish" a continent, it's story more or less stops, leaving tons of wasted potential in its wake.
Take Northrend for example: homeland of the dragons, the ruins of two long-dead empires, the headquarters of the Titans on Azeroth, and a prime spot for colonization now that The Scourge has been dealt with. I feel like there's so much more we could be doing here, but the zone is locked at level 70-80, with no reason to ever go back. I'd love to see an expansion focus on Azjol-Nerub, or the dragonflights, or even a continuation of the Horde-Alliance war.
So what do you guys think? Where can (or should) future expansions take us? Let me know your ideas.