Punk would fight him... yeah.... if he was an MMA fighter. Punk is a wrestler though no matter how you put it.
BUT
Wouldn't that be awesome if Punk turned into the American Fujita or even THE GOAT himself SAKURABA and just started submitting guys in real shoot-mma fights!!!! Sure Lesnar beat a fucking 4000yo Couture and Shane "I-Fought-3-Times-In-4-Years-Than-Retired" Carwin, but that's not nearly impressive as the guys Punk would be fighting around his natural weight.
185-205: Jon Jones, Glover Teixeira, Rashad Evans, Shogun Rua, Chris Weidman, Vitor Belfort, Luke Rockhold, Jacare Souza, Lyoto Machida, etc....
NOW THAT WOULD BE SOMETHING to come back to wrestling too!