So anyone notice that in the final releases of major games in the series, that there is a secret boss that is a major part of the next major game? I'll show you.
Kingdom Hearts Final Mix: The secret boss here was called the Unknown. He wound up being Xemnas in KH2.
Kingdom Hearts II Final Mix +: Secret boss was an armored keyblade wielder, who was later discovered to be Terra from BBS
Kingdom Hearts Birth By Sleep Final Mix: Secret boss was another unknown, who wound up being Young Xehanort, a main antagonist in DDD. Also, you fought Monstro at Pranksters Paradise. Another Hint.
Kingdom Hearts Dream Drop Distance: No final release as of yet, but when there is, I'm sure will get our hint at Kingdom Hearts III.