First idea:
Lasers! :) Everyone likes lasers.

My laser idea is that each tank will have a lazer gun that will do damage if fired at an apponent. More damaged for the longer it is kept on the other tank. The laser would have a battery so there is a limit to the use of the laser and it can recharge. A fully drained battery takes 3 minutes to reload, so don't use it all at once or face the consiquences. Lasers if being fully used will run for 10 seconds. Snipers will like this one; the laser will do more and more damage as the target is further and further away. Objects will of cousrse disrupt the lazer and stop it. The light tank will have the nozzle in the middle of it's tri-barrel gun. The medium tank will have the cannon right beside the brain since the jar always leans slightly to the right. The heavy tank will have the gun come out of the little matinece flap on its front. A button on the keyboard should be able to toggle the cross-hairs for the secondary on and off.

Tell me what you think XD

Cool idea but kinda kills the point of bullets :)