As some people have mentioned, a slowing/drugging needle gun would be amazing... however.
A medigun that has an area-of-effect component? Just imagine picking one target, say, a heavy. That heavy gets 100% of the normal healing bonus if nobody's around. If others are within its (small) range, he gets say, 50%, and the other 50% are spread out amongst the others near the heavy. Overheal and uber would go to the target as normal, but not the others (unless it only overhealed them, but very, very slowly).
That, or some method for the medic to heal himself. That'd be awesome.