Punk's training with the Gracie's and love for mma should be highlighted to tell the story if this happens. I don't see any issue with this feud, if psychology is even semi used here Brock selling for Punk when needed wont be an issue.
I've said from day one DBry or Punk would be great feuds for Brock just due to the size differences, mma training and contrasts.
-P-
Along with this, people should understand that a Brock/Punk match will (hopefully, if they're smart) in no way resemble these nonsensical back and forth matches between Brock and 44 year old, untrained in combat sport, semi-retired desk jockey Triple H. They'll probably resemble Brock's match with Cena more than anything, with Brock using his strength and wrestling ability to throw Punk all over the ring, with Punk getting his shots in due to speed and craftiness.
While it's not the
optimal match that Brock could have skillwise (it's close, but I would still want to see Brock vs. a face Bryan due to the skill matchup), Punk has an understanding of both MMA and Wrestling that's probably only surpassed by Bryan himself. He won't undermine Brock outside of the ring, and he'll make sure to have a match that highlights Brock's strengths inside it.
If this actually does happen (and really, I'd rather it not happen as soon as he comes back...maybe Survivor Series or even Wrestlemania if Brock/Rock has been abandoned), it will probably be fantastic. With the people involved, I have no worries.