#3 John Cena turns heel after losing to The Fiend
They said that it would never happen, but I think that if there were a chance and an occasion for John Cena to go heel, it would be at his upcoming match. He is clearly winding down his career and if he doesn't go heel now, he never will.
So, let's assume that The Fiend manages to defeat John Cena and as he celebrates his win, something changes in John Cena. Cena picks himself up and then attacks The Fiend, inflicting pain and punishment upon WWE's brand new supernatural being.
Heel John Cena could be a very interesting opponent for someone like Drew McIntyre and the storyline of him going for his 17th World Championship could play a big part in that particular feud, I feel. This is provided Drew McIntyre becomes WWE Champion but if he doesn't after the build, it would be the wrong move from WWE.
Plus, we've seen babyface John Cena for so many years now that something like this will be a welcome change.