As others have mentioned, Taker is an obvious choice and one that I'm sure a lot of people have wanted to see and have hope to see since Lesnar came back to WWE. After all, when you look back over the course of WWE history, Lesnar is really the only guy who ever seemed to have Taker's number. In the few times they faced during Lesnar's brief time in WWE, Taker never scored a win. Lesnar had help in some of his encounters with Taker, via Paul Heyman, but Taker's match against Lesnar at No Mercy 2002 in a Hell in a Cell match was awesome and Lesnar scored a clean & decisive victory over Taker during his time as the "American Badass" character. So a I think a lot of people would be interested in seeing this feud ultimately be revived if, for no other reason, than to see Taker balance out the scales between the two.
The problem, however, is that Brock Lesnar is one stiff son of a bitch and Taker isn't, physically, the same man he was a decade ago. The past 4 or 5 years, we've all seen with our own eyes & read various reports on how the wear & tear of the business has finally caught up with Taker. Not only that, it's caught up to him with a vengeance. I don't think many people believe Lesnar has the ability to put on a competitive match against Taker without damn near crippling him in the process. That is an extremely relevant & legitimate concern no matter how many people would like to see this match. Lesnar is extremely physical but I've yet to see him really demonstrate that he has the ability to make a wrestling match LOOK physical without it really being physical. The match wouldn't be at all worth it if Taker got hurt. Entertainment is entertainment, but I don't want to see someone banged up, especially someone like Taker, when he's in there with someone who either doesn't know how or doesn't really care to protect him. Guys like Punk, Orton, Bryan, Ziggler & Cena have shown the ability to do that, but I've yet to see it from Lesnar.
If not Taker or Rock, I think there are some viable options with very over full time guys on the roster. Punk vs. Lesnar II is a distinct possibility if they spread those matches out over a period of months as they did during Lesnar's feud with Triple H. I think another possibility could be Daniel Bryan. WWE has used the David vs. Goliath scenario with great success when it comes to Bryan, and portraying him as a very sympathetic but scrappily fearless underdog against "The Beast Incarnate" could work very nicely. I think it could especially work if it's a WWE Championship feud. There's talk of possibly putting Cena against Taker at WM XXX and if WWE ultimately goes with that, then they'll almost surely have him drop the title between now and the Royal Rumble. Given how hot guys like Bryan & Punk are right now, either is a viable candidate to take the title from Cena. They could also book Lesnar into a match against Dolph Ziggler for the WHC if Ziggler is ultimately able to score a successful babyface run with the title. If he can garner a strong response & level of interest out of the fans during his title run, then he could be booked as a potential money match for Ziggler.
If it's just me personally, I'd rather see Taker vs. Lesnar, Punk vs. Lesnar II or III, Bryan vs. Lesnar, Cena vs. Lesnar II or Ziggler vs. Lesnar over Brock vs. Rock. I think the overall workrate of those matches would be far superior to Brock vs. Rock. Also, I don't see WWE being able to escape the fact that Brock vs. Rock would be seen as the ultimate class of part timers; a match between two guys that are truly, 100% only there because they're getting a shitload of money. Would it draw? Almost certainly, but it'll risk being an underwhelming match in my opinion, especially if The Rock's cardio isn't up to snuff.