Simple, take away the Medic regen and they will consume even more medipacks to heal themself instead of using that energy to heal their teammates.
Medic is also the most endangered class, because in every firefight it’s common to kill the medics first, and dashing across open field to revive that downed Engineer attracts bullets quite a lot, too.
Unless your idea of a fun class is beeing a mobile healthpack dispenser who stays behind everybody else to avoid beeing shot at (and no, I don’t think that you’ll find too much support for that idea ), leave the Medics their regen, its better for the entire team.
To further elaborate: The main concern of every player is to stay alive, because a dead player cant do anything. This is most important for the Medic, because every other class can hope for a revive if a Medic is nearby, so the longer a medic stays alive, the more his team profits from him beeing around (revives/health)
If the main concern is to stay alive, than everything that makes the medic weaker will result in him eating more of his own healthpacks to get to status quo again, which will leave less healthpacks for his team.
The best medic would be an invulnerable one, as he could use 100% of his energy to toss out healthpacks to others and he would never hesitate to dash across the battlefield to revive you.
The least usefull medic is one with low health and no regen, as he would consume healthpacks at a far greater rate, leaving little to nothing for others and would almost never dare to revive you, unless you are lying in a completely safe spot, as he would die from enem,y fire on his way to you otherwise.
Now we have to balance these two needs, making the medics survivability too good creates balancing problems, making him too weak creates teamwork problems. In my experience ET hits pretty close to the sweet spot, the medics survivability is good, so he can afford those suicidal dashes to revive fallen comrades even while under fire, yet they arent unkillable. I like the class as it is now, and if it fits into ETQWs class design balance wise… all is good