Tomko has been one of the main names thrown around. He's also the first superstar on this list whose return would be to serve as Christian's bodyguard.
Problem is, I don't see Tomko coming back to the WWE. He is not working for any wrestling promotion, which works in his favour, but of all the superstars on this list, he is the one furthest from the WWE bubble.
Also, his return as Christian's bodyguard would be far too reminiscent of Christian's CLB gimmick.
Skip Sheffield (edited in by demand)
Admittedly I had forgotten about Skip Sheffield. He has had poor luck with injuries since he was in the Nexus.
While not too much of Skip has been seen, he definitely had potential. He has a good wrestling look, his mic skills aren't half-bad (could use improvement), and most importantly for this list, Has the potential to be an excellent bodyguard for Christian.
Like so many others his role was diminished in Nexus, however I believe he'll have better luck on his next run, should WWE give it to him (see: Michael Tarver).
The potential for Sheffield to run in at SummerSlam is apparent, he has stated in recent interviews that he is ready to make his return "sometime soon".