How about someone who's done prison time for killing someone. Imagine New Jack (except with actual talent) with his three or however many 'justified homicides.' This person could either have a mean streak or could be trying to redeem themselves, especially if their murder wasn't so justified (maybe it was cold blooded and they got off on manslaughter or something, ala American History X, but without the racism.)
A former drug dealer or member of a gang would be interesting.
I'm digging the idea of a greedy Jew that Dethfvk mentioned. Imagine a Dibiese or JBL or Del Rio like character who happens to be a Jew.
How about a sexual gimmick where a guy randomly cums himself (like when he wins a match) uncontrollably, and his entrance music could even start off with him yelling "Urrghh, I'm cumming!"
And what about a Scientologist gimmick?