Back on topic, Mysterio's near the end of his career so turning him heel is pointless. Cena turning heel is nice for a few months but then what? He gets stale as a heel and we all look like idiots for wanting to turn him heel in the first place. The WWE has had chances to turn Cena heel in the past and now I think it's just too late. The only hope for a Cena heel turn now is a temporary Michaels/Hogan style turn before Wrestlemania.
As for stale characters that I'd like to see turn heel. Kofi Kingston comes to mind. Wouldn't mind both Kingston and Bourne turning tbh since Bourne's act will grow old soon enough. They need heel turns more than Cena does.