I can't imagine that a Reigns vs. Cena feud would stay face/face. There's nowhere near enough to capture interest the way that Rock vs. Cena did, and even that feud ended up with Cena looking like an asshole by the time the match happened.
I'm sure WWE will try to sell it as face vs face but the fans will force someone into the heel role by default. They'll either boo Cena in favor of the "fresher, younger" Reigns, or they'll resent the hell out of the possibility of WWE using their ace in the hole of a clean John Cena loss (or even a potential Cena heel turn) to Make Roman Look Strong.
Or it'll be 20 minutes of booing both guys and CM Punk chants.
I know a lot here will hate it but I'd do it at WM33, put the US title on Cena, keep the WWE Title on Roman and neither man gets pinned or submits until then. Around that time Cena gets a little cocky, a little bit like Hunter around 22 where he doesn't fear Roman because Cena's done it all, puts the US Title on the line as well and then at Mania, Roman demolishes him. Not like the Lesnar/Cena Summerslam match but make it clear Roman is the now while Cena moves to a part time role. And I know people will talk about the crowd reactions, you don't worry about that. As long people are watching, let them cheer and boo how they like.
Bout the inner argument going on, I'll say this: I put up with all the Star Wars Prequels cause I was a fan and it was shit. Could I have bowed out after TPM? Yeah. But I didnt and in part only makes TFA that much better.
Same with E. Fans stick around through the bad times for the good times, which though rare, are pretty good.
Except me. I bowed out a year ago and just don't watch anymore.