Doc
Ok, please forgive me if I should already know the answer to this question, but what would be wrong with the original Doc from Fraggle Rock? He is still alive right? Is he medically ill? If not, I think he would perform the job well.
Also, I agree with others that Christopher Lloyd would perform a wonderful Doc. Whoever plays Doc, however, I just hope they study all of Doc's clips thoroughly because Doc is one of my favorites on the show.