In kayfabe, Brock has no reason to want another match with HHH and in kayfabe CM Punk isn't anywhere close to Undertaker's radar never mind on it.
It's not about Brock having a reason, it's about HHH having a reason and if they go with Lesnar/HHH, it'll be somehow HHH pulling Lesnar into a match, probably luring him in to "finish him off" since he failed to at Summerslam. Of course, it would lead to a HHH win, they'll be a tied, and if Lesnar re-signs, we'll probably be "treated" to the rubber. And for Taker/Punk, once again it's not about Punk being on Taker's radar, but rather the appeal of the streak that lures Punk in and knowing that if he ends it, he'll get the respect he desires. I mean, kayfabe-wise HHH wasn't even on Taker's radar for Wrestlemania 27 (no one in specific was), but it ended up happening. If you're talking credibility/star-power wise, you may have a point, but there is no one else for Taker if they go with the obvious rematches of Rock/Cena and Lesnar/HHH. Besides those guys, Punk's their best bet unless they want to build Orton back up, but at this point he's been booked just as weak if not weaker than CM Punk, and he's a face. Ryback still has ways to go despite being the most credible superstar based on build to put the streak in jeopardy. I think in a couple of years if Taker goes on past WM30, Ryback would be a good option if he maintains the aura he has now. Sheamus doesn't have the star power or exposure Punk has right now and isn't as credible as Ryback so that's a no-no. And no one else is really in the realm of possibility.
They could always have Taker take Wrestlemania off, but at the same time his streak has been too big a part of Wrestlemania these last few years where having it in a lower position than it's been the last few years is better than not having at all.
Anyway, it just seems obvious to me they're planning for the two big rematches from 2012 in Rock/Cena, with Rock winning the title and Cena getting a shot in some way, and Lesnar/HHH based on HHH's last promo. Taker/Punk or a Taker match isn't as much of a lock for me, though with him supposedly training for Mania, who knows? But it should be Punk/HHH and either Taker/Lesnar and Rock/Cena II or Taker/Cena and Rock/Lesnar.