As far as the value resorts go, I think they could definately use the princess/pirate(haunted)/magic etc. themes some previous posters suggested. They could make it all work by making a new complex with three resorts, kind of like the all stars.
For a mod, I like the idea of a california surf theme. I know they've done something similar in DL, but I still think it would be neat.
For a hafh, I think a Greek theme would be neat idea. They could style it after the villas in Mykonos and have one of those real rowdy restaurants where they throw the plates on the floor. I'd also suggest an Italian theme, but that might be copying Universal.
For a new deluxe, I'd love to see an asian theme, as someone else already said. I believe they were originally planning one, but had to back out because the location was too swampy or something. Another theme that would be cool is a Chesapeake Bay theme, but that may be too similar to the Y&BC. That would also be a nice mod theme, I really think they don't have enough mods.