Face/face is the way it'll be. Bryan might have an extra dose of cockiness - which I'm hating because it makes me think they'll job him and use the loss as a way of Bryan being humbled and realizing he's not ready to win the big one just yet - but the fans won't boo him and I'm hoping WWE doesn't use this as a way of making it look like Cena is literally giving someone an "attitude adjustment" by defeating them.
If he DOES win the belt, regardless of how cocky he is or isn't, I think it could be an Edge-like interference similar to what happened at One Night Stand 2006, where a guy who has beef with Cena comes and cost Cena the belt but does it more so to hurt Cena than help his opponent. That was how baby face RVD become champ there, and then RVD went into a feud with Edge himself before dropping the belt to him and fucking off (because of getting caught with drugs in real life), leaving the feud to be between Cena/Edge. With Bryan, whoever costs Cena the belt, I could see Bryan entering into a feud with them afterwards because of Bryan being pissed that he cost him the chance of a lifetime to prove he could defeat the #1 star John Cena cleanly. I don't know who would defeat Bryan for the belt but hopefully he wouldn't drop it as soon as RVD did (to be exact, RVD was supposed to have the belt longer but the drug charges forced them to take it from him.)
I mentioned in another thread about this story line possibly having Vince doubt that Bryan can prove he is one of the big dogs of his company and also doesn't even want him to win anyway. if anything, he looks forward to Bryan losing to John Cena to prove his point. Triple H argues otherwise, believing that Bryan can indeed pull off a victory and show that he's worthy of holding the same belt that so many greats have held over the years. He could even say to Cena that no offense, but he wants Bryan to win for that very reason. Remember at the 2005 Rumble when Raw GM Eric Bischoff came down to ringside and started cheering on a Raw guy to win, and the Smackdown GM Teddy Long came down to ringside to cheer for a Smackdown guy to win? I could see Vince and Triple H doing the same thing here in the last ten minutes or so of the match (or hell, maybe they're in their respective corners for the whole match.) Maybe the semi-dirty finish with Bryan winning is Vince trying to interfere on Bryan's behalf but he hits Cena or accidentally trips him or something, allowing Bryan to seize the moment and become champion. I'd be fine with that sort of finish, because the only reason Vince would likely interfere in the first place is if it looked like Bryan might have the match won and his interference only ends up costing Cena further.
Either way, I'm hoping if he wins the strap, they go with a mostly clean victory. I don't see him making Cena tap but some sort of pin fall variation is something I'd be fine with, maybe a small package since it's my understanding that was his finisher in ROH (would be a nice wink wink to his ROH fans who have been following him for years.)