considering we havent seen unverses outside of BBS, it's safe to say that they won't return. (at least in a sequal to KH2) I don't care if there is a new enemy or not.
Well consider that they even decided to create a third enemy and many designs at that possiblily hints of the Unversed return. After All this is Square we are talking about they can find a reason to bring them back, heck maybe in BBS they'll forshadow a rebirt of Unverses