Surreal Life or not, there's no way you'd ever see Chyna return to DX
Cryme Tyme, Flair, Carlito, Shelton, Cena etc. none of these people would be a good addition to DX. IMO I don't see any of them fitting in with the group at all.
IF x-pac comes back (which I don't see happening but you never know), that would be a nice addition. For one, it would be a big shocker since most fans wouldn't expect it, and WWE hasn't had a shock like that in a while.
But if they were to take somebody within WWE and put him in DX, it definitely shouldn't be somebody who's already over, IE Carlito, etc. It makes more sense to take somebody who needs to be repackaged. Somebody said Joey Mercury and I agree with that. Somebody like Masters or Mercury could be good because joining DX would be a nice push for any lower carder.
Any way however, I don't see ANYBODY being the 3rd member of DX. I don't think there will ever be a 3rd member. DX was selling enough t-shirts as it was with just HHH and Shawn and Vince probably felt that bringing in anybody else wasn't necessary. They are probably keeping DX as a tag team rather than a stable, same with Rated RKO (sure Kenny tagged with them but still isn't a member of the group). And at this point, with HHH injured and Shawn rumored to be asking for time off, I think DX will sooner die before it brings in any new members.