Punk VS Austin wouldn't be a good idea. Unless WWE plans on turning Punk heel again, this match wouldn't be the best option for Punk next year. And a heel turn wouldn't make sense, because WWE put so much time and effort into Punk's face turn last year. He received a MONSTER push last summer, and Punk did main event his fair share of pay per views. Also, Punk just had a great match with Jericho at Wrestlemania. It was one of the more memorable WWE Championship matches at Wrestlemania, and WWE would have to do a complete 180 with Punk, if a match with Stone Cold takes place. WWE has put so much energy into building another top face, and Punk has a unique character. You can't expect Cena to stay in the world title picture forever. Having new blood (or a fresh face) is necessary, and Punk can become that guy.
Stone Cold Steve Austin is praised as a God by the mass majority of wrestling fans, and they will not boo him in a feud against CM Punk. Hell, they wouldn't boo him under any circumstances. He could give a Stunner to a four year old girl, and they would still cheer him. The Straightedge face versus the beer drinking, ass kicking, legend, who loves to curse? The fans would eat Punk alive, because Stone Cold will always be their hero regardless, and Austin doesn't have any flaws in their eyes.
Also, and I'm probably in the minority with this one, but I really don't see the appeal of Austin VS Punk. They should be able to deliver a good match together, but for me, Punk VS Austin doesn't have that "must-see" feeling. Punk hasn't reached that megastar level just yet, and for this feud to work, Punk would have to be a heel, because the fans just won't boo Austin.
As far as Lesnar VS Rock goes, It'll be a big time blockbuster match, and the WWE Championship could make the match feel more special. Lesnar VS Rock should draw tons of interest, because both men have come a long way since their match at Summerslam 2002. Lesnar rose to the level of a legit MMA World Heavyweight Champion, and Rock has become a noticeable star in Hollywood. Lesnar and Rock are more than capable of delivering a damn good match, but what can both men offer as WWE Champions? As Jack Hammer said, Rock only pops up once in a while, and you can't expect him to be around on a permanent basis. Brock has a contract, but do you really expect him to stick around after Mania 29? I don't expect Lesnar to stick around for another four or five years, and he won't have any sort of memorable reign as champion.
Undertaker VS Cena is the match I want to see. Taker can still go in the ring, and as long he takes his usual long break, he should be in good physical condition Wrestlemania 29 rolls around. Randy Orton could be considered as the threat to the streak. He's one of the bigger names in WWE, and he did give The Deadman a tough fight at Wrestlemania 21. Kane also makes the cut (mainly because of his history as the kayfabe brother of Taker), but Cena is the man who could be VERY believable as that one legit threat to the streak. Taker VS Cena at Wrestlemania is one of the very few dream matches left in pro wrestling, and I would love to see this match up next year.