CM Punk's self-righteous "I'm better than you because I'm straight-edge" gimmick has already been done,and IMO it's played out,and since Punk and Austin have on many occasions talked about how much they like each other's work and admire each other,it wouldn't make sense to go with an angle where they suddenly hate each other. Plus,for Austin to do what Jericho has already done would be repetitive. I wouldn't want Austin disrespecting Punk's beliefs because their feud won't be personal.
If I were to do it,I'd have a special Raw episode where Austin makes a return. Austin comes out near the end of the show and talks about how he misses the energy and the love he gets from the WWE fans. The fans obviously chant "ONE MORE MATCH! ONE MORE MATCH!" and Austin says that his neck's been feeling good for a while now and he thinks he really can do one more match. Then CM Punk's music hits. He comes out and they banter a little before Punk says "Let me get to the point. You're arguably the greatest superstar in WWE history,and I am the best wrestler in the world. How about we give the people what they want? Stone Cold Steve Austin vs. CM Punk at WrestleMania!"
Austin ponders for a bit,then says "If you want Stone Cold Steve Austin vs. CM Punk at WrestleMania,gimme a HELL YEAH!" *crowd goes 'HELL YEAH!'* "OH HELL YEAH! Now,normally I'd hand you a beer so we can have a toast,but considering you're straight edge and all,I'll just shake your hand." The two of them shake hands,Punk leaves the ring and Austin throws back Steveweisers to end Raw. Then in the weeks leading up to WrestleMania,they air videos showing Austin training and getting back into fighting shape,talking about his neck,etc. Meanwhile Punk does his thing,winning matches,doing a couple of promos about how much he respects Austin,but he wants to prove that he is better than Austin in the ring and at WrestleMania,he will. They have a couple of face to face confrontations where they try to verbally one up each other. Punk will say something like "You can train all you want,but the fact is that you haven't been in the ring for so long,and I'm in the best shape of my life. Frankly,I'm afraid you're going to be out of your depth at WrestleMania." Then Austin looks pissed and tries to go for the Stunner but Punk catches his leg and kicks Austin in the head. "Too slow,old man."
On the go home show,Punk is in a match with someone and pulls off the win,but when he turns around,Austin is there and boom,Stunner on Punk. "Looks like the rattlesnake's got some life in him yet!" and that sets the stage for WrestleMania.