Re: 100% speculation but what if...
It would be amazing and possibly one of the best returns ever.
I wouldn't rule it out completely. Punk/Austin is on the cards for at least some time and they've already planted the seeds.
It's been reported by Austin himself and guys like J.R that he's recovering from that surgery he had back in July last year and if he does appear it won't be until WM30 but they could just be bullshitting to make no one suspect that he'll make WM29.
It'd be the best kept secret ever though. Rumble would have to be on last though, otherwise it'd be pretty clear Punk is beating Rock.