"Backstab deal no damage"
"BATTERIES!"

I like it.
It gives it unique-ness.

Anyways, how about this:

+ Crits might cause a chain-reaction to nearby enemies, most likely to do so with wet players or when submerged in water. (the enemies that got hit with the chain-reaction will only get mini-crits)
+ 15% faster firing speed
+ 100% crit chance againt wet players.
+ Mini-crits has a chance to stun players.
+ Face-stabs has a chance to cause electrocution.
(which will make the victim's aim shaky and will randomly stop movement for a split second, lasts for 5 seconds)

- 95% damage penalty.
- Does not kill critted or backstabbed enemies.
- Distinct backstab sound.
- Crits and mini-crits may cause electrocution to wearer and victim if used underwater.
- Friendly fire to wet allies.

(i) Hold attack button for mini-crit.
(i) Batteries, (and what i changed in my twisted mind)
10 batteries.
Limited.
Can only be recharged by ressuply lockers, except the last 2 batteries.

The last battery recharges like an invis-watch but slower (like 13 seconds or something if fully depleted),
the second battery can only be recharged by fallen weapons and ammo-boxes.

Here is how they work:
1 Crit or backstab, takes a whole battery.
Face-stabs takes only a little bit of the battery, making it so that you can only stab 10 times before the battery is depleted.
Mini-crits takes 2 of those little bits, making you only stab 5 times before the battery is depleted. (only 1 out of 5 of those mini-crit stabs can stun)

Which is bad since you have a limited amount of batteries.
Face-stabs has a 25/50 chance to strip your opponents to melee,
or making them spontaneously taunt.
Mini-crits also depletes 10% of an uber-charge, regardless if they're uber-charged.
But that won't stop them from being invulnerable, it only depletes the uber-charge so that it will end faster.
Mini-crits will also be able to deactive sentries for just a few seconds.
So if you have a team ready to push, just sneak up behind that engie and give that sentry a little stab, and see the engie's face watch in horror as his face gets bashed in by a scout.

Backstabs and crits doesn't really kill you're enemies though.
Instead they'll be forced into a ragdoll state and
have the "you will respawn in X seconds" screen,
except the title is changed to "you will regain consciousness/wake up in X seconds".
After the seconds are up they will wake up where their ragdoll is at.
the ragdolls can't be killed though, unless thrown over cliffs or out of the map or into map hazards. To which they will just respawn normally.

Well other than that, i got nada.
So tell me what'd you think, is it stupid? or unbalanced?

But nevertheless,
fancy exit.

Ad finem