I am going to start off by congratulating myself on writing what could be the 1,000,000th article about a potential John Cena heel turn.
Cena just won the Money in the Bank ladder match, which gives him a contract that he can cash in for a WWE title match. This was the first Money in the Bank match that Cena ever competed in, because he was pretty much always the WWE Champion already. But now, Cena is on the other side of it and can cash in the briefcase whenever he chooses.
So, when he will cash in the briefcase?
In the past, Money in the Bank winners have cashed in the briefcase at times that were most opportune for them. Choosing to do that can come across as "heelish" and as doing it the easy way. Both of those things do not describe Cena and his character.
Could the WWE use the briefcase as a final chance to turn Cena heel?
If Cena cashes in the briefcase soon, it is most likely going to be against CM Punk. Over the last year, Punk has emerged as arguably the biggest fan favorite in the WWE today—which would probably mean that if Cena takes the belt from him, he would be hated even more then he is now. The WWE and Cena could then run with the wave of more hatred and finally turn Cena heel.
This also could be a possible way for Punk to turn back to a heel, which is something he has hinted at recently.
Another possible scenario for Cena to cash in the contract and turn heel could be against the Rock.
After beating Cena at WrestleMania 28, the Rock has said several times that he will be WWE Champion again. If that does actually happen, Cena could get some revenge.
The Rock could potentially win the title at WrestleMania 29. It would be a crazy way to end the night if Cena came out and cashed in the briefcase after the Rock won the WWE Championship.
If Cena takes the title away from the Rock the same night that the Rock wins it, it would drive many fans insane—similar to what would happen if Cena beats Punk.
Realistically, the WWE has passed on many opportunities to turn Cena heel, and he probably will never turn. But if we stopped speculating and hoping for it to happen, that would be no fun.