Heel Cena being absolutely overbearing, annoying, insulting and disgusting could work with a face Roman. Cena would really need to be dastardly, I think he could do it and infact would enjoy the role change. There's a lot Cena could do to make people boo him. Roman is in a tough spot but it can be improved with powerful precise planned storylines that gain him genuine sympathy and also people wanting him to win. Him in a match for Shane for example against a company man could have worked as people would have wanted him to win for Shane to take control. They could have even gone with Cena vs Reigns there and either and possibly had Cena cheat and align with Triple H, Vince and Steph on the show and the next night explain and insult passionately on Raw and cocky as he says he's the best star ever, above Rock (who he beat), Hogan, Austin etc. And Roman comes out fuming and steaming, no smiles, Cena gets away but whoevers with him gets a brutal immense beatdown. Roman had to be shown as a beat and peope to root for him, not so much smiling, not so much promos unless they are going somewhere but show his good points and maybe soemthing so that people can get some background info on dude and create an attachment to him, all together this could help...
What if Cena comes back at mania, but cost Reigns the match, shake HHH hand and the join the Authority? Him and HHH becomes the new 2man power trip. You got Roman over as feeling sorry for him because he keeps getting screwed. HHH still champ and Cena heel turn
. Everyone's happy
it's not supposed to be good