Why not a Disney films theme? We could have different movies in different lands. Leave Main Street and the Castle unchanged (please [-o< !!!), and instead just expand on the existing theming....
Here are some ideas:
Alice in Wonderland in Fantasyland
We could have a nice (and complete) refurbishement of the Laberynth, and expand some of the decorations throughout Fantasyland. Some giant flowers, some "eat me" "drink me" decorations here and there (but not too much)... More Alice in Wonderland music playing throughout Fantasyland (and the Carousel), and of course all the characters!
Peter Pan in Adventureland
We already have Captain Hook's pirate ship setting the tone... Maybe add the Indian's village (or use the one by Frontierland, we could have a nice transition between the two lands) and have Tiger Lilly and the other indians there. Again simply use more Peter Pan music and characters throughout the land, and maybe bring back the Peter Pan show they used to do in Captain's Hook ship?
Nightmare Before Christmas in Frontierland
I love Frontierland during Halloween, so why not expand the theme? We already have a cemetery which could be slightly re-themed to fit with Halloween Town. And for the other decorations, add other characters from the movies: the witches, the werewolve, the mayor... Simple photo locations, again no need to over do it. Or maybe just concentrate the theming to the area by Phantom Manor, so we don't loose the main theme of Frontierland. And the Nightmare theming should really stand out at night - make it even spookier!
20,000 Leages under the Sea in Discoveryland
Discoveryland already has a very strong Jules Verne theme, so they should just expand it a little bit - and they could tie it with the new movie.