If Punk returns to WWE and there is zero drama surrounding him id be shocked. There is always a chance of him going back but I can't imagine the headache of having him there being worth it. They would have to treat him with kid gloves. Part time, limit his interaction with others, watch over how he is treated for pain or injuries, etc.
Also, part of Punks "attraction" is him going against the grain, being his own guy, doing his own thing, fighting the system. He would be doing the opposite of what he was viewed as before if he crawled back to WWE.
To me this is different than a legend who has a grudge with Vince/WWE and later doing business with them again.