I can see him wrestling for another 4 years like he is now, after that he is going to slowly drift out of the company for another 2 or 3 years.
Like he won't do Smack Down and Raw... Just Raw... Then no house shows... Then not every single Raw... then a paper view here and there... Then retire at some Wrestle Mania match. I can't see him turning heel at any point, though I would love to see it. He will be the same John Cena he is now... Just working less and less then retiring for good, I am going to say, by 2020