IMO, Undertaker vs Brock Lesnar is unlikely. Why? Because I'm not so sure that taker can still go with someone who's as intense in the ring as Lesnar is. Brock has a very intense and vicious moveset and manner in the ring, during his first WWE run and even more so since his return because of his UFC background. IMO, Taker needs to be in the ring with someone that can nurse him through a match safely, and Brock Lesnar isn't that guy.
It's a match that I'd like to see happen, but one that IMO is unlikely now.
Punk vs Lesnar is definitely an interesting one. I too can see your plan panning out, and it's one that I've played out in my head too. This is a way to turn Punk back face if they so choose to, and him vs Lesnar has some potential to be a great feud with Heyman and Lesnar vs Punk.
Punk vs Undertaker is another match that I'd like to see. They've had great matches in the past, and could do so again on the grandest stage of them all. It's got potential to be built up really good and it's a match that would produce a good battle in the ring.
Which one of these scenarios is the most likely to happen I hear you ask? I'd say probably Punk vs Undertaker. I see that they've kind of distanced Heyman from Punk rather than continue with the storyline that Heyman hired The Shield to take out Rock at EC leading to Punk questioning why he did that. So I think that Punk vs Taker is the match that's looking likely right now.
PS. This year is definitely not Taker's last match. I think he's dead cert to compete at next year's event given that it's a big anniversary Wrestlemania, and he'll face John Cena there. That, IMO, will be his last match. I also think it's likely that he'll get a HOF induction next year too.