Not going to happen for a while. Considering the rate at which they make skins for champions who have only one release skin, a Thresh skin probably isn't due for half a year at best.
Eh, I'd expect him to be ahead of others in the queue for those with only one skin, due to his popularity (more people playing him means more people may be willing to throw money at Riot). It's not like they always stick to release order or anything. --- Posted from my Xbox360 with 360FAQS v1.1