I was watching a Cain Velasquez career pack when I thought, who can beat Brock, we have a number of upcoming Heavyweight fights which are essentially "Title" elimination fights, many of the winners could argue their way to a title shot.
Randy Couture vs. Big Nog @ UFC 102: If Couture wins this there is a lot of money in a Brock vs. Couture II rematch, I can't see it going any differently, I think Brock would probably defeat him in the 1st round this time. If he faced Big Nog I think he'd crush him and overwhelm him to, there is a very good chance the winner of this will get a shot, but I don't hold out much hope for either guy.
Junior Dos Santos vs. Cro Cop @ UFC 103: Now we all know Cro Cop appears to be past his best, but an impressive win here and I could see an attempt of feeding him to Brock, the name still has some value in it and UFC is about booking the money fights. If Dos Santos won that'd be his 3rd straight win and I think its unlikely he'd get a shot due to his inexperience and not many people knowning who he is.
Shane Carwin vs. Cain Velasquez @ UFC 104: Now I see this as the TRUE #1 Contender fight. Both these guys have been on a tear, I think what works against them is this fight isn't until October and there is also more "Money" in Brock vs. Cro Cop, Couture, Nog with them having more established names. Looking to this fight its a tricky one. Cain is one hell of a wrestler, watching his fights he doesn't have that knock out power and I think he'd struggle with Lesnar, in many respects Lesnar is just a bigger version of him. In many respects I think Cain long term may benefits from dropping to 205, he just doesn't look that big for the weight. Carwin is more your big brute, he has scary knockout power and from what we've seen a damn good chin. I think Brock would look to take him down and control him on the ground, Carwin could well be the man to test Brock's chin and see if he can really take a good shot.
Other Contenders:
Frank Mir's rumoured next fight is against Cheick Kongo. Mir via 1st minute submission I'd say. How many wins until he gets a rematch?
Ben Rothwell. A very good guy in the IFL, he is finally in the UFC, will he end up a contender?
TUF10 Winner: Is the future champion on this show?
Note: After writing this I read that it is official "supposedly" that the winner of Carwin vs. Velasquez will get the shot, I guess it depends on what shape the winner is in though.