I use Medic-- I prefer maneuverability.
I'd probably suggest making Medic's slightly faster, since I've heard alot of people raging that 'Mandos are "OP".
Or perhaps the addition of a "Scout" class that embodies your robot idea, with an increased walk speed slightly above the Medic; minus the heal bonus.
And I've got to agree with Lord-- I found Defender to be useless.
Then again, that could be my favor for maneuverability. I couldn't justify the "increase of HP" to be worth walking the speed of a slimed-out Medic.