I think Fedor or Overeem would stand a chance against Brock. It'd just be a matter of signing them to the UFC. As far as people currently in the UFC go, maybe Shane Carwin?
Carwin is one of the few people that are Lesnar's size and as dominating. He is 11-0, and none of his matches have gone past the 1st round. I mean, this would be a hell of a match. Carwin is coming off a win over Gonzaga. He faces Cain Valezquez at UFC 104 in October, and the winner faces Lesnar.
Lesnar is a much better wrestler in my estimation, but Carwin has more knockout power. If it goes to the ground I would think Brock would win rather easily, but if Carwin would knock him out standing up.
True, very few people can out-wrestle Lesnar. Could make for a classic battle. Of course, Carwin has to beat Valezquez first.