I know what you're thinking, WHAT!? But here's why. In Dream Drop Distance, the hidden boss was Julius, a monster created in a DIsney Short called Runaway Brain, where Mickey's brain is placed inside a Frankenstein esque Pete like monster and now has to recover his body, which is hosting the monster's brain. So, Julius, in a way is Mickey Mouse, and every hidden boss in one way or another, relates to either the final boss or a major boss of the next game. I don't think Mickey is actually going to go bad, but I think after seeing how powerful Sora has become, he'll want to put Sora to a test, and that test being that Sora has to fight Mickey.
Perhaps Mickey will try to trick Sora into thinking he has fallen under Xehanort's influence, so that Sora will have to fight. However, once it's over and Mickey is defeated, and orders Sora to strike him down and put him out of his misery. Sora however refuses, and says he will find a way to restore Mickey. Mickey then drops the illusions by saying that Sora passed a secret test of character. By showing refusal to take Mickey's life, Sora understands that the Seekers who are being controlled by Xehanort can be restored.