First off, I LOVE IT!
Second off, you may as well call it Assassin wars.
Nobody uses any other class? Why? Because permanent invisibility is awesome! And not being able to shoot is no big, just sprint to your opponent.
Here is my idea to fix this:
Either make invisibility timed, so you can only be invisible for maybe a minute at a time. With a 10 second recharge.
Or make it so you are only invisible while you are pressing the V key, therefore making it impossible to sprint to your target.
I would personally prefer the first idea.