Another entry into your expanding costume closet should be the Labcoat. Got invited to a party Friday AND Saturday? You can't go as the same thing to BOTH of them; that would be gauche! Enter: the labcoat.
From the humorous to the horrific, established characters to spontaneous creations, the lab coat is a really flexible costume piece. Mad scientist is the first obvious choice, but should you make up your own or reference someone people already know like Dr Frankenstein, Doc Brown (from Back to the Future), Dr. Horrible (the Joss Whedon creation)? Dr. Stranglove, Bunsen Honeydew or Beaker, Dr. Hackenbush(any Grouch Marx fans out there?), Dr. Jekyll, and Dr. Octopus are all options depending on how much time you have.
Maybe you want a more historical figure like Einstein, or you could make a political statement and go as health care reform (pantsless? shirtless?). Make up your own medical doctor character and spend the night handing out prescriptions ("drink two of these and call me in the morning") or baggies of jellybean "pills". This leads us into humorous ideas which include the proctologist and gynecologist gags that have been popular the last few years.
Surgical accidents can make for a grusome theme with blood spattering and spare bodyparts tucked in your pockets. You could carry around a jello brain and hand out samples ("Oh, he won't be needing it anymore"). Or maybe your dentist is scary enough.