I would advise against it. He could successfully play Mid in a casual match, but it would be extremely difficult to pull off against an experienced enemy. If you come from other MOBAs, Assassins and similar classes are often viable options for Mid Lane, so it seems like Thanatos could be a good option with his strong ability damage. However, for the same reasons I recommend that he plays a supportive role in Joust, he would struggle in Mid. His wave clear isn't as strong as most mages, and his effective range is significantly shorter than mages. That wouldn't be too much of a problem in and of itself, Thanatos can beat mages in Solo Lane after all, but then you have to consistently worry about the Jungler, and Thanatos simply doesn't have a good escape.