Since I hate the idea of Avatarland, I vote Carsland!
Now, the publicized theme of Avatarland I can get on board with (conservation and the like), but they can do that using "real" world examples far better than some fantasy planet. After all, what would work better to get people to think about those issues... their backyard, or some non-existant far-off moon?
I do agree that AK needs some expansion and traffic flow fixes and Carsland wouldn't work there (it'd be perfect for DHS, which also needs some expansion (especially in family rides) and traffic flow fixes (Sunset Blvd to Animation Courtyard for example)). So, in reality, it would be "NotAvatarland" in AK followed by Carsland in DHS.
While I don't mind the Indy/Star Wars sections of DHS, I really think Disney needs to put more focus back on their brands of stuff. A single M&G for their new movie just isn't going to cut it, sorry. The parks are built to advertise and bring to life the Disney classics (characters, movies, shorts, TV shows, etc). Now, I don't mean tossing Mickey ears on everything, but bringing it back in like by using themes, backgrounds, and stories from their more recent properties seems like the best idea.
Of course, if they do that, they won't have James Cameron giving them a bunch of money to do it..