As a "Final Mix" pack, I think at minimum a Critical Mode; new superbosses; new enemies (if only a handful); and additional story cutscenes seems reasonable. As for more high in the sky requests...
-An expanded Twilight Town including the interior of the mansion and areas omitted from the original KHII version (the clock tower, for instance)
-Expansions of certain Disney worlds (*cough*Arendelle*cough*) as well as story scenes that elaborate on events that took place off-screen that the game kind of relied on you knowing the events of the original films to fill in the gaps.
-A fully explorable Land of Departure; if not a very large one
-Expansion of either the Keyblade Graveyard or Scala ad Caelum into a full-fledged final level (which KHIII really doesn't have)
-The return of Radiant Garden and the FF characters to tie up remaining loose ends there like Rinoa making her appearance; what happened to Zack; and clarity on what the history and relationship between Cloud, Sephiroth, and Tifa since KHII only raised more questions than it answered
-Something regarding being able to play as other Guardians of Light (including the three late-joiners); even if it is its own mode or DLC episodes
-Finally, either the last hurrah of the Olympus Coliseum, or (and I would honestly prefer this at this point) an alternative that serves the same point of putting you up against gauntlets of enemies combined with exclusive bosses and fights against FF characters... although for this game, putting you up against both the Seekers of Darkness _and_ the Guardians of Light could be an workable and probably more feasible alternative.
EDIT: and an explorable Destiny Islands; although I doubt that we'd ever get new areas until KH4 - if even then. Maybe have hidden challenges inside the secret cave.