Very much doubt Austin would work with Lesnar. He doesn't seem like the type of fella to come out of retirement, without a reason for doing so.
He seems to like Punk, and wrestling Austin would do more for Punk, than it would for Lesnar, who'd probably just pack it all in, not long after the match.
How would an Austin/Cena build up work? Would we have Cena playing the "I respect you so much, Steve" card. I think it would be pretty painful viewing.