We will probably see a Cena heel turn, but nowhere near soon. It is always a risk on the merch sales etc to turn him heel at ANY time, but it there is a chance to a heel turn, we will witness it when the next face of the WWE will be right there to take that spot.
Cena is a big name that presents the WWE pretty much everywhere. He visits sick children, goes to interviews etc etc. A heel turn will probably jeopardize a lot, in the sense that those "sick children" will lose their hero. Unless someone else, some other hero, comes up, Cena won't turn.
Which pretty much begs the question of who and more importantly when will that next hero emerge. There are a lot of guys who seem pretty talented for that spot. Looks aside, Bryan is already liked by most fans of all ages, he is really good in the ring, decent on the mic and connects with the audience. He is that guy that children like because he resembles some sort of friendly figure and adult fans like him because of his top tier ring work. Other names are possibly any Shield member, in my opinion Seth Rollins.
As I said, more importantly is when and here lies the real problem. Will Cena be around for so long in order for that next hero to emerge? They had a great commodity in Punk, but WWE blew it. They have something great in Bryan, but hard work caught up to him. They are trying to establish new guys now, but they gotta be careful. Pushing too soon isn't ideal, stalling someone's push to avoid that isn't ideal either. I can't see anyone ascending to the Cena status any time soon, which means that at least for that long, Cena will remain to what he has been the past 8 years.If he is still around by then, and said new hero has emerged, then we will most probably see a Cena heel turn and again most possibly see Cena feud with said hero over jealousy about being on top.
I don't wanna point fingers or something, but I think they tried to see in some ways how people react to Cena being some sort of heel with Bray Wyatt. Obviously Bray was the heel, but he tried to rely on taking people with him to support him in his quest, accusing Cena of certain things, thus trying to make Cena the bad guy. To some extent, it worked I believe. People followed the buzzards. We might see something like this blurring the liens between face and heel the next months, since Cena is possibly gonna be involved with the Authority and Bryan. Nothing clear though. It will take at least 2 years from now to see a fully fledged Cena heel turn, if ever.