Honestly, what I would love the most (but is extremely, extremely unlikely) is a revamp of Outland, just to show what it's like 10+ years after the events of TBC, the Legion getting dismantled and all the other things that occurred.
Same with Northrend, but not as much.
Not saying I want an expansion built around that or anything, but it would be sad if the TBC zones were left out of a zone revamp yet again.