It's been confirmed before that once a person of strong nature loses their heart and becomes both a heartless and a nobody it is possible to bring them back if you destroy both their nobody and release their heart. This is apparently how Xehanort will be resurrected after Terra-Xehanort was consumed by darkness. That being said, if Riku and Sora and everyone else do manage to slay Xehanort, wouldn't the cycle just start over again?
It's been confirmed before that once a person of strong nature loses their heart and becomes both a heartless and a nobody it is possible to bring them back if you destroy both their nobody and release their heart. This is apparently how Xehanort will be resurrected after Terra-Xehanort was consumed by darkness. That being said, if Riku and Sora and everyone else do manage to slay Xehanort, wouldn't the cycle just start over again?