Only way to really do it so everybody wins.. is have either Batista or Brock cost Cena the match
Then Rock wins... Cena saves face and has his next big dollar feud.
That to me would be terrible. This build up has been going for 12 months. To end it like that would be horrible.
This deserves a clean winner... simple as that. Its rare at Mania that a main event match is Russo-ruined like that.
If Cena loses it kills this generation of Wrestling that is already considered (and lets face it) is not as good as Rock's era. Thats what a Rock clean win does.... but even that is better than a dusty ending.
It would be unfair to Cena to do that IMO. Rock had his WM matches with Austin that he'll always be remembered for. This is for Cena basically because he is leagues bigger than the next superstar created from the post attitude era (like him or hate him).
If Rock wins fine... that might push the door open for a Cena heel turn. Who knows. But IMO it should be clean.