CM Punk seems likely. Supposedly, Brock doesn't want to work with Punk (don't see why he wouldn't) but we'll see. Heyman becoming displeased with Punk and calling upon Brock to destroy him or them forming a very short-lived duo that goes awry quick is a logical enough set up for them to fight. Maybe Punk isn't returning full time yet, but rather, whatever happens at Payback or after creates tension between him and Heyman, which leads to Brock returning and putting Punk out (possibly without Heyman's consent, similar to the F-5 on Vince earlier this year) for another month or so. Punk then returns pissed, etc.
I want to see a Randy Orton feud too, which is why I don't want Orton turning heel. I like the image of Brock being hit with the Rope Hung (second rope) DDT, or Orton going for an RKO only to be thrown halfway across the ring while in mid-air, or Brock kicking out of a punt kick and then delivering one of his own to Orton, etc. Orton could tap into his Viper persona a little more and really pull off the kind of intensity necessary for a feud with Brock.