Would he still had been the leader of the New Nexus....
It depends. Would it have been a personal feud, or would it have been two wrestling technicians going at it? If you recall, Orton took on and took out each member of the New Nexus leading up to WrestleMania 27. Would Bryan have done the same thing? I doubt it.
If the plan was to go with Punk vs. Bryan, I'm guessing it would have been about the wrestling, with less storyline. At the time, Bryan hadn't developed a "character", so it would likely have been about the Title.
Who would have if he didn't?
My guess is that they would have stuck with Wade Barrett. I think creative dropped the ball by having Cena end the original group so quickly. Barrett had truly found a groove as a top heel, and he was pushed aside so that Punk could have a mini-feud with Cena then a larger one with Orton.
But had Punk not come along, I'm guessing they would have stuck with Barrett.
Would he have still got to deliver the pipe bomb?(remember he may have created magic with bryan and not been pissed)
Absolutely. The 'Pipe Bomb' Punk delivered was
part of the show. WWE did everything within their power to convince us it was not, but this was a scripted promo. And if Punk was pissed at the time, how do you think he would have felt, even creating magic, chasing the US Title after being a 3 time World Champion?
Not very happy, I imagine.
Who would've feuded with Orton?
Well, who was left? Perhaps Sheamus, I suppose, since Bryan and Sheamus were paired together. Orton wasn't a big time player on Raw anymore, and neither was Sheamus after his first two WWE Title reigns. Its not hard to imagine, as Sheamus had been in a huge feud with HHH the previous year, so it's not as if he couldn't handle one with Orton.
would punk vs bryan be the dark match like sheamus vs bryan was?
Heck no. You think Punk was mad about his position in the company in the midst of his feud with Orton? Imagine how he would have felt if he was bumped off the card? Besides, Punk was still a big-time name, and even if he wasn't considered main-event status yet, he was still a big-time player. He did commentary while injured, which not many wrestlers can say.
Even if Bryan wasn't at that level yet to create a great feud with Bryan, Punk would have brought him there. And there's no way that match would have been bumped from the card. Punk was a bigger commodity at the time then Sheamus, period.
Exactly where he is right now.