The counter button should be the only customizable button in diep.io so that every1 can counter to the best of their ability, maybe the auto buttons should be customizable too, but this is about Counter. When countering, how well the counter does depends on how close the enemy bullet is to your tank.
If right before: Perfect! No damage unless hit by bullet afterwards (if its that slow or big)
If slightly after: Good! You took some damage, now hope the bullet has passed you
2 seconds+ after: Fail! Your tank fails the dodge and when the bullet would have passed you, you end up dodging into it, taking more damage than a normal direct hit.
The actual counter part will only be available with tanks with guns. If the tank does not have guns (as a melee tank would) it will get closer. In the event where a gunless tank performs a perfect dodge, it skips ahead of the bullet, moving it towards the enemy. If the tank has a gun, after the dodge, a shot that does half the damage of the projectile fired-dodge time % missed= damage of counter bullet. Counter bullets will be be more of a pentagon blue, instead of tank blue or bullet red. These bullets will also be shot very fast and be hard to dodge with anything besides the counter button, creating massive counter wars. For a button as the counter button, I believe it should originaly be "Q". A thrust itself may be used as a dodge, and counter can be just shootig back if wanted instead.