Kingdom Hearts 3: 10 Final Fantasy Characters Who Must Appear
9. Zack
Zack made a brief, yet fantastic, appearance as a teenager in Birth By Sleep, forming a particularly strong bond with Aqua. When she inevitably returns, her connection with Zack would be a fantastic (if potentially tragic) thing to see.
There's a fifteen year difference between Kingdom Hearts Birth by Sleep and the first title, so it's entirely possible that Zack's adventure has already come to a close, which would also make sense considering that Cloud is wielding his buster sword.
Despite this, we don't really know where Zack is in the Kingdom Hearts universe. It would be an incredible opportunity for Zack to make his appearance when he's grown up and help out against an attack on Radiant Gardens. Nomura has previously stated that Kingdom Hearts 3 will feature a "darker" story, which means a sacrifice from Zack isn't entirely unlikely.
The best case scenario would be having Zack survive in the Kingdom Hearts universe, but it seems unlikely for Nomura to make this sort of change.