Agreed - it's a perfect opportunity fot Cena to turn heel.
But I believe, as I've always believed since the announcement of their match at the next 'Mania, is that this year will be the last year (not ever, obviously) that John Cena will be face.
With WWE slowly becoming more edgier since The Rock's "return", but still maintaining PG, I believe that after their proposed match at Wrestlemania - which I believe The Rock will win, whether it's for the WWE Title or not - that John Cena will FINALLY (no pun intended) turn Heel.
What will happen, in my opinion, is that John Cena will lose to CM Punk come Sunday. Vince McMahon will come down to the ring and fire John Cena, only to be met with a kick to the face by Punk, then AA by John Cena. Cena and Punk then shake hands and Cena let's Punk have his moment with his hometown crowd. As Punk cuts another great promo, he is attacked by Raw Money in the Bank winner Alberto Del Rio. Del Rio defeats Punk to win the WWE Championship, instantly making him a mega-heel. Vince awakes with an evil smile on his face. PPV over in shocking fashion.
After this, the proposed Cena vs. Del Rio Summerslam main event will happen which will kick start a great feud between them. Ultimately, John Cena wins the WWE Championship back and takes is to 'Mania with him - but this will be Cena's LAST face run with the WWE Championship.
As I said, with WWE becoming more edgier, I think WWE is extremely slowly preparing fans for a possible Cena heel turn.
So, come 'Mania, The Rock vs. John Cena for the WWE Championship. Fans, as expected, cheer the HELL outta the Rock, and most Boo for John Cena. (Let's face it, it's going to happen)
The Rock wins the match and his 7th World Title. At the end of the match, John Cena shakes the Rock's hand and celebrates with him...then gives him the AA and just walks out.
On Raw, the next night. Cena cuts his first ever heel promo in years about how he is sick of all the fans not accepting him as the future of the company and that he expected everyone to get behind him as 'Mania the previous night - But instead all the fans went back to the past - namely the Rock. John Cena then says "Screw Hustle. Screw Loyalty. Screw Respect. Screw the WWE Universe" - instant heel turn, and WWE becomes EPIC again!