I think Cena is fine as a face.
Cena shouldn't turn heel, and here's why.A face Cena can be toned down and quietly put on the backburner, but if Cena has a big shocking heel turn, then a large chunk of the show is going to be focused on "Why Cena Why?" and focusing on the motivations for his heel turn. If Cena turns heel, then he'll be back to being all over the show.
The reason why Cena gets the hate he does isn't because of his stale character or whatever. It's because he's been presented as "The Center of the WWE Universe". The heel turn is only going to make him the center of the universe all over again.