This may sound highly skeptical, but doesn't it seem kind of odd that the shield HASN'T attacked Cena yet? They've beaten up ryback, Orton, Sheamus, Bryan, Kane, and the Rock, but not Cena. Also Punk keeps denying any connection. Wouldn't it be great if Cena is the one directing the Shield? Would be a great swerve and generate a ton of interest. Still leaves a lot of faces with Ryback, Sheamus, Del Rio, etc.
Cena turning heel is like the great pipe dream of the internet and it's just not happening, sorry. Cena's winning the belt from Rock at Wrestlemania as a babyface and he'll go on to feud with heel Orton for three months and maybe another feud with Big Show or Brock or heel Sheamus or whatever. Besides, I wouldn't want Cena to turn heel and have a group of hired guns doing his dirty work because Jesse Baker booking coming true in real life would just be WRONG.