I had a similar thought myself the other day. What if Cena loses to The Rock again? Would that be enough to turn him heel?
Personally I don't see how he'd turn heel at 'Mania, short of teaming up with The Sheild - that would suck. But the following night on Raw?! The internet would explode, rating's would go through the roof for a week or two and then probably nose dive. Still if WWE starting polling the same numbers as TNA we really could have a new Monday night...scuffle(?!)
Short answer is: hell yeah I'd be excited if Cena turned heel but probably not for any of the right reasons.