If this gets a Final Mix they'll probably throw in a "future" secret boss so I'm not even worryin
Or in the International version. Mysterious Figure was never in the original BBS in Japan. He was added to the international version of the game, the first time that ever happened as usually the future boss battle is in FM only.
Granted, 3D FM will get ANOTHER secret boss on top of that, canon or not.
And it appears that all secret bosses for the next game are the penultimate boss for the next game.