Human Thresh Skin [Ultimate Skin]
According to his lore page, Thresh was a human who was charged with guarding the vault buried beneath the citadel. By spending so much time around the dark energy he became more and more corrupt, even developing a taste for torture. By the time the Ruination happened, he had become so corrupt that he became the spectral wraith as he appears now.
The simplest version of this skin would really just be a human model, depicting how Thresh might have looked before the Ruination.
However, I think it would be an even cooler, maybe even an ultimate skin, if Thresh starts out as a fairly normal looking human, and as he gains levels, or as he collects souls he grows more and more corrupt. I would not actually have him reach his full spectral form in a match, but it would be cool if you could see that he is on the way toward that.