Bryan's smaller style and underdog character are tailor-made for a babyface. I can't see them turning him anytime soon if they want him to successfully cash in the MitB case. The only scenarios I can see him turning heel in are:
-He becomes the first Money in the Bank winner to lose his title shot. He goes apeshit on the defending Babyface Champion, beating the hell out of him after the match and simply becoming a Machine thereafter. He arrives, dismantles his opponent with an array of headlocks, counters, and a submission finish, and leaves. There's nothing fans hate more than a heel countering a cool offensive move into a headlock. Trust me, people hate headlocks. He could essentially fill the role of Chris Jericho. Not that he's the same as Jericho, but I could see him playing that same type of role.
-The other scenario involves him winning the match against a heel champion, prefferedly Christian. He would come out the following Tuesday (hopefully live) and absolutely bash the commentators, people in the back, and finally the fans for not believing in him. He would bad-mouth the fans for turning on Christian and claim that he's turning on them.