The only thing, if Orton DOES turn heel, is that he needs to do it without the monotonous talking and "kick you in the...SKULL!" bullshit. Just be Randy Orton, the guy who got on the radio and called Kelly Kelly a whore. Be THAT guy.
I don't think they can actually turn him heel by having him do something to Cena. Bryan, however, appears as if he could turn Cena himself Heel whether Cena likes it or not, so they could always use Bryan to turn Orton... I just pray it isn't a short title-reign for Daniel Bryan. Kinda feels like, "We'll see just how much of a reaction Bryan gets as champ and as soon as we're displeased with him, whether we give him a shot or not, Orton will take it off of him."
Best way they can use Orton with MITB though, is to have him just come out and RKO the champion every week or so in the ring, in the back, during entrances, etc. just to be a total prick and set up that he can cash it in outta nowhere. Shit would rule!