Ryback before he apparently turned heel.
I remember last year thinking it was possible for them to go this route with Wrestlemania. One of the endings suggested for the HIAC match at the PPV of the same name was Heyman calling upon Brock to help Punk retain the title. This would have planted the seeds for a Ryback/Brock feud. Ryback is a legitimate opponent for Lesnar that people can buy into, and Ryback shell shocking Brock and defeating him cleanly would have been a huge feather in his cap.
I'm not a big Daniel Bryan fan but I can imagine an underdog match between him and Brock. I absolutely would not want to see Bryan defeat him, but if it was booked in a "How long can Bryan last?" way, it could still do wonders for Bryan. Bryan is small, but you can play up how internally, he's one of the toughest guys in the company, perhaps every much as tough as Brock is. He can also his technical prowess to his advantage. Brock ultimately wins, but Bryan still looks daring and tough for his effort. The thread title is Who Do You Want Brock To PUT OVER, not who you want Brock to lose to. Booking it this way would do that. He would have to drop the YES stuff for this feud to work, though.