It would be better, nowaday to release a "Final Mix" version of Kingdom Hearts 3 as a season pass, rather than buying the Final Mix version as another game.
The problem with a "Final Mix" DLC to add to the original game would be the coherence. The Final Mix version of KHI and II added some cutscenes, items or boss fights in the main storyline. Thus, it would be strange to add some items, bossfights, cutscenes that trigger during the storyline with the DLC, since, most people will finish the game before any DLCs comes out I guess, missing all the "Final Mix" things.
The advantage of Final Mix contents being sold within a new copy, is that you have to start from the beginning, you can't miss any added features. (Another positive aspect would be, that we're getting another front cover for the physical copy )
So, you'd have to pick a new save file0 and start the game all over again. I can't think of a game that added story elements within DLCs that force you to start a new game.
Anyway... I'm more focused on how they will fit the extra content rather than what I would like to see in it.
At this point it would be hypocritical to criticize extra content in Kingdom Hearts, for the "Final Mix" versions, are what we would call today "Game of the Year Edition", "Director's Cut Edition", "Definitive Edition", and so on. I love what the several Final Mix versions brought to their original game, so... What I don't want to see though is a ton of little DLCs for skins, music, weapons... I would like to get A "Final Mix" DLC and only this. Or, a Dark Souls type of DLC, two to three big DLCs. But no little DLCs.
As for the content of the DLCs... maybe a totally optional world, one that the fans wanted but could not be in the main game. New bosses of course, with a sadistic, fast as hell, tanky boss, the usual.
Oh, and no in-game pre-order bonuses, of couuuurse.