After playing a few missions, I have a few thoughts.
Try restricting mission types to specific tank classes (light, medium, heavy, td) and within each class, have specific types of missions that reward good behavior for that class of tank.
Example, lights and mediums should be rewarded for spotting enemy tank, or helping sniping teammates spot an enemy tank. Scouting type missions. Maybe shooting out enemy tracks?
Heavy should be rewarded for close in fighting--not sniping. Your system is able to tell how far a tank you kill is from your own tank. I'm sure this will force more heavy tankers into close in fighting.
Td should be rewarded for longer range fighting. Bonus points if they are able to kill an enemy they are able to see because of an ally spotting them.