I like how some people feel Daniel Bryan would be a good candidate to perhaps pull out an upset victory over Brock, but yet feel Punk isn't credible enough to do so.
I personally feel Punk shouldn't win a match with Brock either, but it could easily be a good feud. Punk going face is much more plausible than Brock. The monster that Brock is, especially with his legitimate UFC background, almost anyone that goes up against him is an automatic underdog. Not the kind of guy that fits that well in the face role. The money is in seeing who can actually go toe to toe with this monster and win, so unless they intend it as some sort of squash match, have faith that WWE isn't dumb enough to turn Brock face to go up against a heel Punk.
But anyway, Punk could put up a very good and believable fight and if he did that and lasted a while before inevitably losing to Brock, that would be good enough. I don't like Punk going back face either, but it's bound to happen eventually with the kind of reactions he gets. I even wonder if they could pull off a heel/heel feud with these two. They're difficult to pull off (successfully) but not impossible. Hell, Punk is already a tweener (in terms of the reactions he gets) so they could make him out to be more of the baby face of the feud with Brock without officially turning him, and then have him move right on to another feud with an actual baby face after the Brock feud is over.