There's a reason why they were never booked to have a meaningful feud between each other. Because they sucked together.
Their characters don't mix. It's like seeing Elvis vs. Boris Karloff. Their characters are too far from one another in the universe to create a meaningful feud between them. Even now, if they were to "feud" (i.e., cut a dozen promos with each other, since that's what it would be for Wrestlemania), it would feel awkward and forced. I can think of a few avenues they could take with it--"Team Bring It" vs. "The Creatures of the Night"? Nevertheless, it doesn't excite me any, at all. Let's also be brutally honest: Undertaker was the least among the triumvirate of Taker-Triple H-Michaels this past Road to Wrestlemania when it came to promo work. He received "WHAT?" chants and was even getting boos here and there for some shaky mic work. Seeing him paired up with Rock would probably render the problem altogether more pressing. At least with Lesnar, Heyman can do the heavy lifting and sell the storyline without necessitating too much mic effort from either Taker or Lesnar.
The best thing that ever took place between Rock and Taker was Rock kicking Taker back down with the "Rock Stomp" after Taker "sat up" mid-People's Elbow on a random Raw in the fall of 1998. The rest of the match? Nope. Their match at Survivor Series later that fall? Boring Attitude Era "brawl," nothing offensive but unmemorable in the extreme. Their match at King of the Ring 1999 is actually atrocious, a nearly 20-minute snoozefest. Austin winning the title back the next night had nothing to do with it. By that point that night, I had repressed the match between Taker and Rock the night before because it was just so poor.
When I think of Rock and Undertaker, I think of Rock's December 2000 promo in which he mocked Taker's eye-rolling shtick (though Taker was the "American Badass" character at the time). Again, it was hysterical in large part because of how massive the gulf between those characters truly is.
To reiterate a point I made in a thread regarding Shawn Michaels having one last match--and it quickly was dominated by talk of Rock and HBK working together one day--"...Rock wrestling a guy who will be pushing 50 when they could ever conceivably lock it up in the middle of the ring in 2012, with Rock standing in as the Hogan figure from 2002 that he wrestled once upon a time as the young megastar, is just taking a backward step for WWE. And I'm sure Rock himself knows that. Since returning, Rock has been and continues to be working with Cena and Punk and Miz, not with guys who he either worked with a decade ago and don't need any further interaction with him or even legends like Michaels who he somehow missed out on ever working with. He's a lot cagier and wiser than he's given credit for, seems to me."
I think Rock's own fans aren't giving their own hero enough credit for having such a smart, well-rounded view of the business.
This isn't ruling out Lesnar/Rock as a possibility one day as a simple major attraction to boost a Summerslam or a Survivor Series one day, but at least as far as Wrestlemania XXIX is concerned, it seems like both men have entirely different trajectories. They've made it clear that as far as Cena and Rock are concerned, they have unfinished business with one another (their dialogue at Raw 1,000 saying as much), and Rock's locked in to wrestle Punk, almost surely at The Royal Rumble. Rock freeing himself up for Lesnar would require quite a few tricks in booking at this point. Meanwhile, Lesnar is here solely for big money and to work with the biggest possible names, and after Cena and Triple H, and with Rock busy with today's bigger stars, Undertaker would be next in line for Lesnar.
I'm just going to focus on 1 aspect of this because Rock36 responded better than I could on the history of Taker v Rock.
You don't get over with boring, stale feuds. If WWE's plan is Rock Punk followed by Rock Cena 2 and to continue driving home the same played out arguments of Dwayne left for Hollywood and isn't deserving they're not building anything because they couldn't interest a single person long term. Cena or Punk defeating Dwayne Johnson is not going to help out their legacies. Not going to build new stars.
You have to build based on what's going to be most fresh, entertaining because you just don't get over on wins and losses alone. Entertainment is everything.
Punk Rock at the Rumble followed by Rock Cena 2 is going to be cringe worthy bad. Punk's already laid out his material against Rock and it's safe to assume that it's the same material Cena has. Then you follow that up with the rematch nobody wants to see.
I can't believe anyone thinks this is a good idea.