The two that immediately spring to my mind are Ambrose and Barrett, Ambrose especially. Ambrose is one of the few all-rounders in the WWE, along with Punk, D'Z and Bryan and the latter two could get much further as tweeners IMO. Ambrose has obviously got the mic ability, has a pretty awesome look, really unique gimmick and portrays his character fantastically well in the ring. Barrett just needs a bloody decent gimmick, then he'll go straight to the top. With his presence and mic ability he's born for that role.
Cesaro would probably be my third choice, though I'm not sure whether he's good enough on the stick to be that high up. Still, fantastic look and best worker in the company behind Bryan IMO.