The Taker/HBK point has already been answered so I'll leave that be.
I think it's more than likely that Austin will be competing at either this coming Wrestlemania or the next, in fact, I think that it's almost guaranteed. Rock vs Brock probably won't top the bill because it's two part-time wrestlers competing, so if Austin did compete against Punk that would get the nod because overall, that would be the bigger, and better event simple as.