They just did Detroit! (Mania 22? I think?) Screw that place, it's a hole.
I say New Orleans or Jacksonville. Somewhere new.
I think part of the attraction of having a Mania in a city is also the city itself. I had buddies who went to Detroit, and then went to Miami (for 24) and said they had a much better time in Miami because Detroit is a derelict shithole, that's waiting to die. Miami had a beach yes, but also had a significant nightlife after all the fan events closed down. Wrestlemania is becoming a week-long fan oriented event, much like the SuperBowl, and the city's local nightlife should definately be a consideration in picking where to stage the next Wrestlemania, IMO.