ALTRUISM
"My enemy is a notion, not a nation."
Thematically, it would be compelling as hell if Cena returned and fully embraced being a scumbag, hollywood heel who only looks to cement his legacy in the same place where it started by attempting to finally break Flair's record. 
People'll groan, but the guy who should stand as his obstacle to that goal should be Roman - the same man he also hasn't beaten, which would also add some weight to the feud. Much like Austin, we maybe start seeing cracks and even some doubt forming into Cena. He needs to beat Roman; more than he could ever imagine.
It writes itself so perfectly which is why they won't do it.

People'll groan, but the guy who should stand as his obstacle to that goal should be Roman - the same man he also hasn't beaten, which would also add some weight to the feud. Much like Austin, we maybe start seeing cracks and even some doubt forming into Cena. He needs to beat Roman; more than he could ever imagine.

It writes itself so perfectly which is why they won't do it.
