Yen Sid has been the closest thing to a mentor/master Sora has had, helped by the fact he's the leader of the anti-Xehanort movement. I think it would be really cool for Sora to learn from Aqua as his mentor as well. Maybe he won't officially be her apprentice but I think there's much that Aqua could teach him, and she'd no doubt be grateful to him for rescuing her and be more than willing to teach him. Aqua is awesome, we need more of her anyway.