Im generally interested in the Spy players would like as their options from their weapons as unlocks: if youve no idea what Im on about, the medic is getting two new unlockable options, one being a new healgun that is a superoverhealer with no uber, and the other unlock a new kind of needlegun and bonesaw.
The point is to give players more options, make the game more interesting and in depth for the players who play a lot, whilst keeping the average player with equally strong weapons, and aspirations to unlock the new ones. Some players will keep to the ubercharger, some will change it for the overhealer, and some medics may choose a different offensive power style.
What I would like to see from Spy unlocks would be this:
A revolver that doesnt ever randomly crit, but crits people 100% if you shoot them in the back, where the crit shot would do less than that of a normal revolver (~80-100), so that those times when you're behind an enemy and arent going to catch up for your knife, you can shoot them for a close kill, and finish with some body shots, or take a shot or two in the back and retreat, allowing teammates the kill, or coming back later. Why would I want this? So that I wouldnt need to rely on my knife for instakills at melee range, I could try and take out players in a similar way (getting behind and attacking from there), with more risk, but more reward (I can make the kill still, but Im further away). However the revolver would be weaker for those times when you pop off some shots as a medic chases you with a needlegun for killing his heavy, or other such times when you can shoot people.
Remember, if you are going to make a suggestion, it might as well make sense balance-wise. The weapons need to be equally strong, and give advantages to certain types of play, rather than making the weapons stronger than the originals.

Edit: 02/5/08 (the second of May for the Americans)
Ive gone through the 25+ pages and tried to pick out some of the better ideas, but before that Id like to tell you that vision and movement impairing is not going to happen.
Hellblazer:
"There will NEVER be any more "conc nades" or tranq guns. Valve established early on that they do not want the control of the character taken away from the player."
"Silenced Pistol instead of Magnum. Doesn't reveal disguise but does less damage."
Spoo!:
"best thing i can think of is
prox mines, they replace the sapper.
spy holds 4 max, they are hand thrown with a decent cooldown (2-4 seconds) time after each is thrown. The spy can only have 4 deployed in the field at one time.
Like the sapper, they can be used while disguised without dropping the disguise.
They explode only when enemies make direct contact, they cannot be detonated. They take 5 seconds after thrown to become live, so they are not very useful in open conflict. They do about 100 damage, requiring 2 next each other to kill any class but the heavy (needing 3). The splash is very limited, meaning that only one person will take damage, unless people are on top of each other. They can be blown up by enemy fire by members of the opposing team only.
To the opposing team they look exactly like friendly noncrit demoman stickies. To the firendly team they look different is some way, to let your team know where they are.
They have no effect on buildings."
Doc Brown"
"What about fake medkits/poison vials?"
a-scale:
"I've got another one. I believe it's perfectly in line with the character of TF2.
A kick me sign. That's right, a piece of paper that you can put on an enemy's back that says "shoot me", which makes that soldier look like a hostile player, and turns friendly fire on for that soldier for a limited time. Want to confuse the hell out of a rush? Put the "shoot me" sign on the back of t