I'd put the likelihood of this type of match happening at less than 30%, but with a match of this magnitude with two Superstars in as good a shape as they are in (the year time frame gives Rock plenty of time to train between movie work)....the ONLY proper way to settle this would be an Iron Man match.
Its been a while since we've seen one at Wrestlemania (the only other one to happen at the big event was probably Shawn Michael's single greatest match, against Bret Hart at Wrestlemania XII where he finally captured his first Heavyweight Title).
Cena could do an iron man match, and Rock could get in the shape this next year to do one if he's not already there (physically he is, just would question whether he'd have the stamina). That would be the best, and IMO the ONLY match suitable for Rock vs. Cena at Wrestlemania. Give those two guys the last hour of the show and let them produce pure wrestling gold for the ages.
You can't tell me there would be a better match setting than the Iron Man match.
They could sell out of WM tickets in less than two hours TODAY, without any other matches on the card, if they announced that would be the headline match (Rock vs. Cena in an Iron Man Match)