Fortree City.
Also: Route 110/Cycling Road, Sootopolis, Route 113, Jagged Pass, Mt. Pyre...
Fortree.... c'mon - it's a city of treehouses. What's not to love? And in 3D, we should be able to actually go through and around the trees rather than just over them, and the footbridges should bow and sway and, hopefully, still go "tok tok tok" as we walk across them.
I've always liked the layout of Route 110 and Cycling Road, with the route snaking back and forth underneath the overhead road. That's another thing that should really do well in 3D.
And Sootopolis - first, I can't wait to go through the underwater entrance and bob up inside (assuming we still do that, and since it'll be a great opportunity to show off the wonders of 3D, I'm sure we will). And the city itself should be great in 3D - a ring of terraces and paths looming up on all sides.
Route 113 for the bleak ash fields, just because I'm curious to see how they handle that.
Jagged Pass should be awesome in 3D - steep and rocky and... jagged.
And Mt. Pyre could well be the 3D highlight of it all, inside and out. I really want to see what the view from the top will be like.