An idea for the overcharge would be a laser cut. Instead of a laser pulse, a continuous beam is shot which you can use to "cut" through enemies. The higher the cut angle, the less damage per target. It should have a higher "ammo used"-to-"damage dealt" ratio than a normal shot.
And the shoot from it should have an autohit. You aim and fire with a burst of light, closing enemy with the speed of 3*10^8 m/s. Who can dodge it?
IF you aim correctly. I don't think you can actually dodge pistol shots either, so that effect wouldn't... in a logical... this is only a game... ah, screw this, you know what I mean XD
An idea for a derived unique would be a prototype laser SMG. Since the weapon does not produce actual recoil, high-precision autofire is possible. Essentially, this would be an accurate plasma gun with (far?) less damage and no chainshot function.