I think it would be great to fight the heartless in KH3. Nobodys would be cool, but not TOO necessary. (although we propably will be fighting against Xemnas, Xigbar, and Isa.) I didn't like the Dreameaters too much, and the Unversed felt kinda pointless. (they were really cool though)
I enjoyed fighting the heartless way more than those other ones. (Well, saying that... I haven't played bbs or ddd)