While I tend to believe the general consensus of The Big Show - it pains me to think that he would sooorta have to turn again. I mean, Punk is a heel. Show is replacing Cena. Cena is a mega face. For me, Shows heel turn was too recent and too wholistic (like, he turned VERY heel, not even remotely tweener) for him to just pop back in and be a protaganist.
My solution? Jeeze. That's tough. Who is a big enough face to go up against Punk? I hate Orton, so I'd prefer not him. I think if Henry came back as a face that'd be cool - but we already have Ryback, Brodus, and Kane playing the roles of huge, monster, faces. Maybe Kofi Kingston? They could use the angle that Kofi and Punk used to be (probably still are, but for storyline purposes) friends in the back. Punk has changed. He's not the same person, bla bla bla. Maybe Cena can act as a mentor/coach for Kofi. But I just highly doubt they serve us a Punk vs Midcarder in a Hell in a Cell match. Ahhh. I have no idea how this will pan out if Cena can't make it back.