WWE lost their opportunity to turn Cena heel convincingly when they didn't do it against The Rock. If being booed by everyone for a guy who had ditched the WWE for Hollywood didn't make him snap then nothing will.
That being said Cena's better as a face anyway. The guy is the face of wrestling right now (no matter what HHH and Steph would have you believe
) and that's not gonna change in the foreseeable future.
People constantly talk about Cena turning heel and say that it worked for Hogan but what they don't realize is that by the time Hogan did it he'd pretty much milked the Hulkamania thing dry and NEEDED a change. Cena on the other hand is still marketable as is, and as they say "If it ain't broke, don't fix it".