Here's 3 possible I got:
1. It could be the new Xehenort that comes out after the nobody of Xemnas disappears. (I mean those nobodies with strong hearts can't possibly can't be completely dead... it's disney & Nomura, c'mon people)
2. It's Riku/Xehenort.. i mean we know from what Mickey said to Riku at the end of KHII, "You don't belong in the dark realm anymore Riku." But in the future he still longs for the dark power he had but learns to control it. But in order to go full strength he has to summon his Xehenort look/ability. maybe the KH explosion in KhII didn't erase Riku's dark power... only put under Riku's control again or locked inside Riku's heart, idk. (lol Riku was inspired by terra SO much when he was a boy that he wants to be just like him haha even LOOK like him lol) ALSO: Riku is the only Character in KH that I know of that wears vests that ALWAYS show off his arms... just saying
or 3. it could be an entirely new character just how Vanitas was, but still has connections to someone or something.