We all know that Doctors are almost not needed in the game. Especially when your drivers are maxed.
Just to explain briefly why: It is useless to pay a doctor around 400k per race to gain 3% in training progress and lose 10% form instead of 15...
Maybe, to improve a little their effect, doctors could also allow a gain of form besides to reduce the loss of form in training. So a 5-star doctor allow a loss of form of 10% (15 + 1% less per star) and then the driver automatically regain form by 5% per hour, but with the doctor he could have 1% more per star, so that's make 10% per hour.
I understand level 20 managers with maxed drivers will still not need doctors (unless aged drivers start losing points ? I don't know) but for the others it could maybe allow alternative strategies...