I agree that sora is not a guardian of light. See my reasoning behind this goes back to kingdom hearts 1. In the original game the narration told sora that he would open the door to light. While some may speculate that that has happened, I speculate that maybe sora is the one left after the 13 and 7 clash to form the x-blade. See they will go into the dive to the heart and sora will be left alone with no way to stop anything. Kingdom hearts looming in the sky above when xehanort not risking himself as one of the 13 envelops the worlds in darkness as they once were. This would make it so that sora would have to open the door to light, But he might lose the keyblade. It isn't his to begin with, first of all. Ventus will be his own person by then making that idea disappear and the only other way he had it was because he had to open the door to light. This would leave him with just magic if even that again an average boy.