OK here me out. Everyone has been talking about the three knights and such and I want to focus on the ending with mickey's cameo. What if the secret ending was what caused mickey to disappear in the first place. If everyone remembers mickey appeared in the realm of darkness at the end of KH. We never saw him before then so we have to assume he had been there throughout the game. What if after the scene we see in FM+ secret ending Mickey ends up getting into Kingdom Hearts and after the battle with whoever the true enemy is (prolly the old guy) he then is transported to the realm of darkness where he eventually encounters Riku and Sora closing the door. I know this is a long shot but hey I figured I would share a thought i had