I think they still need to establish the personalities of a few of the current Nexus guys before they start bringing anyone else in. You have Barrett, the smart, no-nonsense leader. Sheffield is the (slightly psychotic) muscle. Otunga I guess is what his nickname says, the "A-list", the arrogant one with the money and the famous wife. Gabriel, Slater, Tarver and Young don't seem to have much character, though Young is out now so I guess he's not much of a problem.
I haven't seen any of this season's rookies except when they've appeared on Raw or SD so I can't really guess how well they'd fit in with Nexus.