Here's the problem: Brock Lesnar left pretty much at the height of his popularity in WWE...pretty much. He had probably the biggest push in WWE history and got more over with greater success than anyone else in WWE history (just my opinion). Had he stayed in WWE in 2004, he could have continued feuding with Eddie Guerrero, or started a feud with Booker T, or John Cena, or Randy Orton, or Batista, and so on. Brock Lesnar was pretty much the last vestage (sp?) of the post-attitude/pre new generation mega pushes (2005, for Cena, Batista, Orton, and Edge). Yes, Orton's push started in 2004, but he didn't become a successful main eventer until he turned heel in early '05. Edge, you could make the argument that he became big when he speared Shawn Michaels at Taboo Tuesday, or when he won his first WWE title.
The point is that Lesnar, like Triple H, Undertaker, and Shawn Michaels needed to stick around, past WrestleMania XX (one era ending) and throughout (into another era). That's what makes guys like that so important to history. Triple H has wrestled The Ultimate Warrior and The Miz. Undertaker has wrestled Dusty Rhodes and Jack Swagger. Shawn Michaels has wrestled the One Man Gang and Ted DiBiase.
Lesnar never became what he could have and should have been. Instead, he became one of the most notoriously fickle athletes in history, who in the last six years has been WWE Champion, kinda played for the Minnesota Vikings, and gotten his ass kicked by Shane Carwin. Yes, he is a big name in UFC. But he doesn't represent it, simply because he doesn't seem to know where he's going. If WWE wanted to do a HUGE WWE vs. MMA fight, they would have tried for Randy Couture (sp?), Chuck Lidell, names who EVERYONE knows, including those who don't watch UFC. I don't watch UFC, but I know who they are and I know that Randy Couture vs. Triple H would be momentous.
If WWE heavily played on Lesnar's history in WWE, then you would have an intensely novel feud: a guy who made it big in wrestling, ditched in, and has now come back. A lot of new fans might not even know of Lesnar's history in WWE, they might not care.
This is what makes pairing him off with an opponent so difficult. Here's the problem for me: I don't want to see Lesnar vs. Undertaker at WM. Why should I? Because UT asked Lesnar a question after he got his ass kicked at the fight, and did it out of character. How could that possibly be a feud? Break kayfabe for this one guy and have UT go American Badass for no apparent reason and betray his character?!? No way. As someone else said, Lesnar vs. The Dead Man would be ridiculous. I, too, am a big UT fan and think it would make him look stupid.
And what kind of match would it be? UT is 45, injured to hell, can't carry a match and can barely be carried to one. Just my opinion, but this was none more evident than at WM XXVI. Lesnar hasn't wrestled in almost four years, and hasn't a good match in almost seven. He's not exactly Kurt Angle.
Also, we've seen this feud: twice. After they killed each other in Hell in a Cell for 28 minutes, there's really not much left to to. Lesnar ain't HBK that they can just resurrect this feud a decade later.
Cena? No, Cena deserves a wrestler for WM, not a "celebrity. Cena works too hard to be overshadowed by a guest star, not on the grandest stage. I don't want to see Cena play second fiddle to someone who's heart clearly isn't in wrestling, even if he does make Lesnar tap to the stf with a chain around his throat.
Triple H? Maybe. That'd be my closest preference. A few reasons. The obvious: they never faced each other one on one. Whether it should have happened from 2003-2005 or in 2011, it's still one of the biggest matches that has never happened. Whether it's Triple H vs. WWE Lesnar or Triple H vs. MMA Lesnar, it's still Triple H vs. Brock Lesnar. They've never fought (aside from Global Warning, which not everyone even knows about it), they're evenly matched, and HHH can still carry just about anyone to a good match. Build the feud around Triple H taking special exception to Lesnar disrespecting the wrestling business, portray Lesnar not so much as a heel but someone who legitimately defends his decision to leave (kinda like The Rock in 2003), and either guy could go over. HHH could beat Lesnar and defend the honor of wrestling, while Lesnar goes home with a major paycheck, or Lesnar beats HHH clean, and shows that maybe he misses wrestling more than he lets on (very unlikely), or Lesnar beats HHH dirty, like Mayweather/Big Show.