Undertaker, Cena, Ziggler, Orton or Brock will win the RR.
First, I think a Taker/Punk match would be the best ME title match they could do. Punk would be credible enough to MAYBE Beat the Streak.
Now, I wouldn't be surprised if they held Cena from getting close to Punk until Mania, and having him win the Rumble to set up the ME.
Think of the possibility, Ziggler, who still hasn't cashed in the briefcase going into the Rumble, wins the Rumble and challenges the WWE Champion Punk at Mania, he wins the title from Punk at Mania, and cashes in his briefcase to win the World title against either Sheamus, Show, or Orton to become the Undisputed Champion.
Only way I think Orton wins the RR is if Ziggler is the World Champion at the time of Mania.
And finally, Brock winning the Rumble. Brock vs Punk at Mania with Heyman as the guest referee. Who will he side with? I could also see Brock vs Rock as well.
Now, this is just a gut feeling. They might take the title off of Punk before Rumble, which would be a mistake. I think that they might want to hold off on Rock vs Punk until WM, and the only way that is going to happen is if either a Punk loses the title or they come up with a situation where Rock gets "injured", maybe by Brock? Then, whoever is GM or Vince, tells Heyman that he knows Paul told Brock to do it for his client Punk, but that Brock won't be protecting the title for Punk, in fact he might be taking the title from Punk at Rumble. And have a Punk/Brock match at Rumble. OK, I am going off of far off plans, just rambling whats going off of my mind.