I think Cena Will return after WM27, For the reason that he is going to be unexpected. Think about this in Vince's Shoes.
If everyone is expecting him back before WM27, and he don't show, Everyone will think he actually left for good. Then, At Extreme Rules or something, Boom! John cena is introuduced and comes back to destroy the competition once more.
After WM27? Seriously? Keeping the biggest draw in the company away from the biggest show of the year is probably not a good idea. Sorry, but that's a ridiculous idea.
I doubt Cena is going to be gone long, if at all. If they do decide to play this out I say bring him back right after Elimination Chamber. Wade Barrett and Nexus are going to have to answer to The Undertaker for what happened at Bragging Rights. I could see Barrett offering Cena his job back by doing his dirty work for him and have Cena wrestle Taker at Mania. The stipulation would be if Cena wins he gets his job back. That would present a greater threat to the streak than anything else ever could. I just don't know what Taker's status is and if he'll be able to compete at Mania.