Every time a new attraction is being talked about, the rumors seem to fall into three general categories:
1) Rides based on movies and existing properties like Star Wars Land or a Marvel character
2) Bringing back an old attraction like Adventures through Inner Space
3) Have Imagineers come up with something new entirely
Personally I'd like to see Disney go the route of #3 and come up with a new, original story to tell. Most of my favorite rides at DL fall into this category (e.g. Pirates, Haunted Mansion, BTMR). I think this gives them the most creative latitude to come up with something new rather than recreating something that I've already seen in some other form.
Rides based on movies are great because they help you feel like you're a part of the experience that you already have a connection to. Retro attractions are fun because they tie into that nostalgist in all of us, but I'm not sure how much some of those classics resonate with kids or average visitors to the park.