Those feuds have been done, time and time again, you said so yourself. With Cena as the heel, things could be much more interesting. Again, I'm not saying it's going to happen, but Cena as the heel would be different, and probably more entertaining (given how many matches these guys have already had against one another).
Much more interesting how? Because Cena will sneak attack people and then run away in fear when they try to get back? How is that more interesting, when it's the same heel tactics we see every week from the plethora of heels the WWE currently has?
How would it be more interesting?
I have no idea what kind of heel Cena would be. I'm not sure he'd be a coward like Hogan and Austin were. Maybe he'd be a complete badass. Who knows. You can say it's not time, but you can't say it wouldn't be interesting.
I can say it wouldn't be interesting, because a) we know what heels do, and there's no reason Cena would do those things, and b) Cena is easily the most interesting character the WWE has.
There's no reason for Cena to go heel in the next several years, unless we see the rise of the next Steve Austin. Until there's a big time face the WWE can hand the ball off to, one that John Cena HASN'T feuded extensively with, there's no reason for Cena to go heel in the prime of his career.
Main point - we don't know where Vince is at with this. Sooner or later, it's going to happen.
Maybe....maybe not. Sting has never really been a heel. Sure, TNA tried, but you see how miserably that failed. The Undertaker was briefly a heel, but again, you see how miserably that worked, and he was only turned heel because Vince had Steve Austin.
As I said, Cena is very close to becoming beyond the idea of face or heel, and he is simply a superstar who does his thing and is himself. Cena can feud with faces or heels, and the WWE has far more room for creativity with Cena's character in its current form.
Heck, look what Cena's character has done just this year. It's made main-eventers out of both Miz and Punk, and is the foundation for the epic dream match at next year's Wrestlemania. The fact Cena plays such a controversial character (as the WWE always says) has far more interest for fans than playing to the traditional face or heel disposition.
But, regardless, it's going to happen, at some point.
Never did for Ricky Steamboat. Or Dusty Rhodes.
Lots of reasons:
1) To see the boyscout become the bad guy
2) To see if you can be evil and still be Super
3) To see if anyone can really fill his shoes
4) Because as much as I like the guy I am tired of him too
5) To hear the hatred of the crowd when both sides join forces in their hatred
6) To see how long the hatred lasts
7) Because he is so talented and will do a great job in the role
8) No more crappy straight to DVD movies starring him
9) It would shut up so many of his haters and all this perpetual Cena heel talk
10) Because kids gotta grow some time
Yeah...not ONE of those things has anything to do with pro wrestling stories. Look at your list...not ONE thing has anything to do with the actual pro wrestling itself.
That's what I mean when I say there would be no point. Everything you listed is related only to off-screen preferences and hopes. I win.
Now Crock, Sly and KB give me your reasons you want him as a fan to stay the way he is?
Haven't I done so multiple times already? Because a heel turn would accomplish nothing, there would be no point, Cena is already the most interesting character on the show, he's already made two main-eventers this year and is building up for possibly the greatest Wrestlemania buyrate in history. Cena's character is flexible in storylines, as he can work with either faces or heels, and have equal credibility in the storylines. His character allows for the evolution of his opponents character, in whatever manner the WWE decides.
I want him to stay the way he is because he's easily the most interesting and developed character in the WWE.