Re: Randy Orton's gimmick isn't for sell.
Well Cena isn't going to turn heel soon so we don't even need to have this discussion
You say that, but he has been a face for a very, very, very long time. He's been playing a fan friendly gimmick since his before his first championship; in fact, he's been a consistent babyface since late 2003. Look at how many people have come and gone since then! Seven years is a long time to go without a turn of some sort. Now would be a pretty good time for a Cena heel turn as there are still enough young fans who idolize him who would be genuinely shocked. It would be like Hogan's turn in 1996 - nobody believed that would happen, either, and it was great for business.
The best way for this to happen is to have this happen:
Have Cena as the one who attacked Undertaker, build up a rivalry that leads to Wrestlemania and then have Cena end the streak! You'd have the hottest heel in professional wrestling.