One of the advertised headliners in Rock, Brock, or Cena. Punk will if he faces one of them, and then obviously there's the possibility of a streak match.
HBK coming out of retirement would suck, and probably isn't happening. Rock/Taker could be interesting though.
Has to be the Undertaker, in his final match ever, against John Cena, in his biggest match ever, have him nail the tombstone, the crowd go crazy, Undertaker celebrates one last time, then, he gets the fair well like Ric Flair did on the following raw.
It should be really interesting bc Taker has never went up against a guy who was "face of the company" like Austin/Rock/sadly enough Cena. WWE would want Rock to job to the streak long before Cena which is utter bs! If Austin returns then obviosuily he will headlined.
Also Flair had a really amazing career and he didnt even headline in his retirement match against HBK, some say he didnt want to but still. I dont think Taker ever cared were he was on the card so he may be 2nd to last.