Plus, after the game clear, you get to pick any partner character as the main character.
But then, the bonus boss is from Kingdom Quest XVI Remake.
That's KH, DQ and FF combined.
I take it that the secret boss is supposed to The Estark of Masters Omega?
Though a bonus dungeon would be nice, taking place in a parallel version of the main game world; it'd also have a powerful superboss at the end.