I think it should be like this

-On stab turns enemies into a ragdoll Puppet
-getting damaged lets user absorb 50 damage
-uncloaks on 50 damage
-disappears on 50 damage appears 20 second later
like the spy-cicle but different

-Backstabs do not instant kill but do a percent of their current health

-spy has two needles, he can achor one into the ground, the other he can backstab enemy players with
-backstabbed players are tied to the anchor and must free themselves though finding it through following the trail of thread (theres a limit to how far the thread can go from its anchor)
-pyros can burn the thread, freeing the tied up players
-multiple enemies can be tied to the anchor

These stats focus more on stalling your opponent, giving you enough time for a getaway or giving you some distance to finish off your tied up enemy.

10% moving speed when active
15% damage vulnerability on wearer when active

Originally posted by Reapox:

-Backstabs do not instant kill but do a percent of their current health

-spy has two needles, he can achor one into the ground, the other he can backstab enemy players with
-backstabbed players are tied to the anchor and must free themselves though finding it through following the trail of thread (theres a limit to how far the thread can go from its anchor)
-pyros can burn the thread, freeing the tied up players
-multiple enemies can be tied to the anchor

These stats focus more on stalling your opponent, giving you enough time for a getaway or giving you some distance to finish off your tied up enemy.

I like the overall idea of this, but I think it might be op if the thread didn't break after a set amount of time.

On backstab:

Target is forced to walk forward (and cannot do anything else) for 3 seconds before dropping dead
+25% cloak meter

Cannot use cloak for 2 seconds

(+) Your last backstab victim will be death-linked.
Death-links will mark the victim for death and make them take ALL damage that you would have taken (Note: Any effects that increase or decrease damage that would have been taken will be applied to this effect, including crits)
(-) Backstabs only deal 10 damage.
(-) Only one player can be death-linked at a time.

Critique would be greatly appreciated. It helps me get better at this...

On backstab players mouth is sewn and they are mute until respawn
Silent Killer
Dead Eyes: Spy only sees in different shades of black grey and white except for blood which is red.
-25% reload speed
Roulette: all revolvers has been reduced to one shot in barrel that has a chance to do half damage or mark an enemy for death.
-20% cloak from ammo packs
Rations: Health packs are not instant but slowly given into your health pool
A team communication destroying macabre weapon that utilizes your knifes strengths over your other weapons. Rations could be abused or the thing that ends you.
Also when you backstab an enemy they go into a cloud of smoke and become some sort of stuffed animal. Scout into a bunny, Soldier into a Racoon, Pyro into a shark, demo into a parrot, heavy into a grizzly bear, engineer into a turtle, medic into a dove, sniper into a crocodile, and Spy into a crab.

Or It could be a Kunai reskin?
But, If I had to give stats, I'dd give It something like;
+ Upon a succesful Backstab, turn an enemy into a Doll.
- Can't use Watches.
- Can't use Disguise kit.
Alt-Fire; Teleport to the last doll and take Its disguise, fully restoring HP.
+ You can use two shots in your revolver without breaking disguise.

Just a reskin of normal one would be nice

+On backstab, turn Enemy into Doll
+Right click looking at the doll to hold the doll to take it's disguise, and leave it as a skeleton.
-If you don't change the doll, a engineer can figure out who killed them and spy on you.
-When holding doll can't use weapon, cloak. You can only chose to options.
+If a heavy was turned into a Doll, It will drop three sandvichs.
+ You can throw the doll at a enemy which will change the enemy to be killed. (Into a skeleton)
-50 Minus health
+On backstab, earn 50 health back.
-If cloak, You'll drop the knife. Making you have to wait 1 minute untill getting another knife.
+If lower than 50 Hp. Any shot from the back turn's the enemy to a doll.

Any problems with 10 Suggestion's? Or any problem with one of these ideas?
Lemme know!

Oh, One more
+You can sew the ground and make a hole to make enemies fall in the when facing hole with enemy inside, Alt+Fire will start to close it up and then the enemy will get crushed throwing out his doll.

On backstab: Player's team is switched lasting for 5 team kills.
On backstab: Victim is allowed friendly fire
If victim does not achieve the 5 team kills then their team is switched back after 5 minutes
HIDDEN: Victim's model's team is not switched

How about on backstab the victem is disguised as you and will walk towards enemy players, however they cant do damage and die in one hit

Ok so... a lot of people were suggesting things related to puppets BUT, i thought of something a bit different, so these are the stats.
(+)On backstab-The victim faints for one second, while the spy auto-cloaks, after the victim wakes up, they are blinded for 5 more seconds and suffers the bleed effect, the bleed though, does 5% of the players current health and will not stop unless cured with an healthpack. (Medics and dispensers do nothing)
(+)When a victim is actively bleeding, the spy gets health back at a rate of about 30% of the damage the victim is taking with the bleed, the spy gets the health evety second, in accordance to the victim taking bleed damage.
(+)After making 3 people bleed to death the spy gets a "blood-surge" which he can activate when he wishes to, the blood surge refills the spy's health to its max instantly.
(-)Any Damage you recieve during the victim's bleed will do double the damage and give the victim health at a ratio of 10/1 damage taken by the spy to the health given back to the victim.
(-) +10% cloak time when manually cloaking. (auto-cloaking after a stab will take normal time)
(-) +15% vulnerabilty on wearer.
Welp, thats it guys, thats what i thought of! btw.... i liked the design ;P