It would boost ratings if there were a strong enough opposition to battle Cena. Even if the Rock were to sign back with WWE to a "Lesnar-type" contract, it still wouldn't be enough; they would need him full time, which will not happen. Who would a heel Cena battle in-between for the interest in the character to remain? Face Sheamus vs. Heel Cena isn't going to happen unless they unify the titles and Sheamus is no long the "face of SmackDown!" (sorry Randy). Orton isn't the "Savior" type of character, Rey Rey has been out of the hunt for a while. Ryback isn't ready. The only "possible" lead for someone to face Cena would be Miz... I haven't seen enough of his face turn to fully invest in a role reversed angle between the two. This would require a HUGE amount of faith in Miz, which I don't see the E Universe having the amount require to pull the trigger on this angle at this moment in time. My point - without enough "stand out" face characters in the WWE, they can't take their one stand out and flip him. It would be of less value and would do the opposite of increase ratings.