#3 Royal Rumble win, WrestleMania main-event
CM Punk's desire to be in a WrestleMania main event might be completely dead at this point, but that's probably the only promise WWE could make to him that would get the former World Champion to make his long-awaited return to Vince McMahon's company.
The best way to bring Punk back would be at the Royal Rumble event, where he could enter as a surprise at number 30, which would get a massive reaction, especially if WWE were able to keep his return a secret. This would mean that he'd have to win the match, and the go on to challenge whoever the current WWE or Universal Champion is in the main event of WrestleMania.
Aside from a Royal Rumble win and a WrestleMania main event, there probably isn't anything that WWE could offer Punk to get him interested in returning.