Adventureland is the land with the most expansion potential ideawise, in my opinion, but sadly is in a location that makes such expansions difficult. My idea is for a method of expanding Adventureland (which would also remove the much-despised Tarzan's Treehouse attraction-a wise move, as Tarzan is not really raking in any money for Disney anymore and the attraction is particularly uninteresting). My idea is simple-remove the Tarzan theming from Tarzan's Treehouse, making it into a more generic treehouse. Now expand the treehouse network massively so that it allows access to a new subland behind Frontierland, Deep Adventureland. Add some lore about the trive that created the treehouse network, and possibly work it into the new attractions in Deep Adventureland.
I acknowledge that my plan is rather vague and requires more elaboration to be truly useful but I think that it is a good way of expanding Adventureland to contain new imaginative attractions.