Nicholas Lyndhurst? Well, I did see him as Uriah Heep in David Copperfield, and he might work. Does anyone thing David Tennant is too handsome to play Rincewind? I mean, he has the running and thin thing down.
Other alternatives? Well, someone mentioned Mackenzie Crook at one stage, didn't they? On this board, I mean. Or maybe Bruce Spence?
I think that David Jason was a good Rincewind in that he had the personality if not the looks.