They are really thin on faces not just on RAW but in general. If Cena tuned, I don't know who they would have challenge him unless they do a double turn and Miz becomes a face.
Damn, that's a tough call.
There are plenty of fans who want to boo Cena, but there are just as many who want to cheer him, and neither side "needs" any more reason to do either respectively.
I think the biggest challenge is getting everyone on the same page.
It's unusual for a main-event, company posterboy to receive such a mixed reaction from the fans to the extent that Cena has.
IMO, to put him in a program with Rock suggests that it is to take Cena to the next level by getting him over in a way he could never accomplish himself.
And, that's not to say it will necessarily be a heel or face push, per se.
This could be a reversion to the "Attitude Era," which saw less focus on the good guy/bad buy paradigm, and more emphasis on "characters."
Bringing back guys like Austin & Rock
(even just temporarily) would certainly make sense as a reintroduction to this format.
Either way, change is on the horizon.