Medic is the most important class who should never take risks going into battle. Unless he spots an equally high value target: the enemy medic.
Option 1:
Weapon that works better on overhealed players. Either by removing it or doing extra damage to players with overheal.
Option 2:
Weapon that applies a heal block effect. The enemy temporarily cannot recieve healing from beams, thus forcing the enemy medic to stop hiding behind heavy's massive body.
Option 3:
Weapon that can steal ubercharge from enemy medic. Perhaps as an extra feature for uberspritz.
Option 4:
Similar to third-degree, hits everyone connected by beam.
Option 5:
A medigun whose uber can force stun an enemy (thereby blocking enemy ubers)