I'm a fan of both of these match ups, and they are much more appealing to me than the match we seem to be getting at this Mania (Brock/HHH) or the match everyone on the IWC seems to be clamoring for (Brock/Taker). Orton would be interesting story-wise if his character could return to the ways of old (and this does not require a heel turn) could be very awesome. Match-wise, he'd have to stay face as heel him, at least during his last heel run, slowed down his offense way too much for me to find him enjoyable.
Also, I find it hilarious that people are legitimately calling Punk/Brock an "unrealistic" match up. I mean, smaller guys can be bigger guys in real fights as there are more strategic ways to fight and entire classes dedicated to teaching people to defend against those physical stronger than you. For example, Jericho beat Goldberg once but I understand that's not a perfect example but it doesn't need to be as that, or what I said in the previous post, isn't my point. I just felt like throwing that out there.
It seems that people who are calling this unrealistic, are under the impression that it will be presented as they are on equal footing but to assume that would be ignoring how Lesnar's been booked up until this point. Build-wise, Lesnar would mostly likely dominate Punk any time anything physical happened. The match itself, would probably play out akin to the series of matches Punk had with Mark Henry over the WWE Championship. Which, if I recall correctly focused on Henry dominated the bulk of the match, with Punk getting offense in at sporadic moments, and normally big high-flying moves or a hard place strike or something like that. I don't recall exactly, should re-watch I liked those matches, but it was presented in the most realistic fashions possible. Hell, I believe Punk only won one of those matches from pinfall/submission, and did so only because weapons were allowed. I see the Brock matches going like that (though hopefully with a different result, Punk shouldn't win this match in my opinion) but if they do it like that, and their's nothing pointing that they wouldn't, I don't see how it's unrealistic. At most, you'll have to use suspension of disbelief a little bit on how durable Punk is to Lesnar's offense, but that's nothing new to wrestling.
Maybe it was called unrealistic for a completely different reason but I can't honestly think of one. It can't be from why the match would happen since it'd make complete sense due to the Heyman connection and probably the easier of the two presented matches to build.