The obvious answer is the beer vs. Straight Edge gimmick ... but that is already being done right now and has been the center of a number of feuds.
It would also HAVE to be a Face vs. Face match, so no reason to try and force someone into being a heel.
It would be very easy to do. Just simply have CM Punk start talking about how HE is the face of the company now and the Champ. How Cena got the shot with The Rock and blew it and now he wants his shot.
Punk could throw it in here or there for a few months about how he wants he shot at "a legend." Mostly with a focus on the fact that he would not "blow it" against The Rock. Have him keep calling for HIS WrestleMania match and have the crowd start getting into the Austin chants. We are not stupid ... we will all know where it is going.
And then you simply have Austin come out ... maybe as early as SummerSlam and say ... you want a legend Punk ... then here I am! Punk can give his wry smile ... accept and then we can get a fun war or words with a few stunners and maybe even a GTS mixed in and take it from there.
These two guys are really excellent at telling stories with words, actions, moves, etc. Get them in the ring together and it will take care of itself.