First off, I don't consider what Heyman has to be a "stable," in the classic sense. It's not the 4 Horsemen, nWo, DX or even his old Dangerous Alliance. He represents two wrestlers - Brock Lesnar and C.M. Punk. These two wrestlers are NEVER seen together, they don't have tag matches, they don't do run-ins to help each other. They basically have nothing to do with each other on screen, but just happen to have Paul Heyman as a mutual friend. So, in that sense, I don't see the "Heyman Guys" as a stable.
But, for the sake of discussion, here are my choices, including one dark horse:
1) RVD - As someone stated earlier, the whole "Heyman Guy" idea is rooted in reality, as both Brock and Punk are friends with Heyman and owe their pro wrestling stardom to Heyman. So, if you'd actually want to form a faction of "Heyman Guys," common sense dictates that the next person would have to be someone that falls into that same mold. I could see RVD coming back to join the ranks. I've seen interviews with RVD posted lately saying he'd possibly come back to WWE. Why not have him come back under Heyman's fold? That would give the "Heyman Guys" a good variety - a big man, a mat wrestler and a high flyer. I know RVD's up there in age now and may not have much left in the tank, but it's a logical choice.
2) Kofi Kingston - I like Kofi, but the dude's in serious need of a repackaging and a heel run. Aligning him with Heyman would be excellent, as Heyman could help get Kofi over as a heel. It would be difficult to turn Kofi right now b/c he's been a face for so long, the kids like him and they brought the whole "I'm a first-time dad now" side out in the public, but I'd personally love to see Kofi go heel for a while. Alligning him with Heyman could help that turn stick better.
3) Daniel Bryan - This seems like a good fit to me. With Team Hell No likely about to split, perhaps it's time for Daniel Bryan to get that edge back. Maybe he finally gets tired of feeling like comic relief or being called "Goat Face" or whatever and snaps. Maybe Heyman sees that change and convinces Bryan that he needs someone that will bring him back to the top of the mountain in WWE.
4) Randy Orton - The heel turn is coming... eventually. Every time we think it's going to happen, it doesn't. Does he NEED Heyman? Of course not. Randy got by just fine as a heel before, even leading his own faction. But, since the fans are so behind Orton right now, having him RKO a top face like Sheamus or Cena and having Paul Heyman shake his hand for the turn could be just the thing to get those boos back as a heel.
4) MVP - Here's my dark horse. There's rumors that MVP may be coming back to U.S. wrestling, now that he's done with New Japan. If he does come back to WWE, why not put him back as a heel and pair him up with Heyman. He could revive the "biggest free agent" angle when he first debuted, but now actually have an agent alongside him in Heyman. I admit that it's a little out there, but I could definitely see it happening.
The next "Heyman Guys" SHOULD NOT be Ryback (his character works better as a lone wolf, bad mic skills or not), Antonio Ceasaro (just needs the right high-level feud and meaner attitude to make it), Dolph Ziggler (already has a faction and is on a roll), The Miz (he's the next John Cena, in terms of being the WWE's poster boy) or The Shield (same as Ziggler).
If you'd like to go another route with this, I could see a few options. Maybe an NXT rookie coming up to the main roster that Heyman sees potential in.