To be honest, I doubt Punk will retire at an early age. He may have the money and the fame and all the required things for an early retirement but I think he loves the business too much and would either miss it a lot or just get really bored without it.
That being said, I wouldn't say it's likely, but I wouldn't say it isn't either. Punk's last heel work was unsuccessful because most of the people who liked him before he had that heel turn liked him from a non-kayfabe standpoint and respected his work - these people obviously don't boo people they respect.
So it seems it would be pointless to make him a heel, and I think WWE know that.