I was also thinking Faster Speed, but I think it would be more of a Laser Weapon.
+25 Fire Speed
+25 Weapon switch speed
On hit on Building-Absorbs 5% of its Metal
-30% Damage.

Like the retro futuristic weapons it fires a laser projectile.
Projectile cannot be reflected.
Damages enemy sappers.
-50% clip size
-25% damage.

Hmm... I was thinking that it would be more for fire support.
+10% Accuracy
+15% Firing Speed
+10% Damage
Oh Hit: +.1 seconds of defuff (on them)
-25% Clip Size
-10% Ammo Capacity

(Shoots Lasers)
+10% damage
No ammo needed
Once killed a spy 5 second crit
Once hit invisble spy cloack goes to 0%

On equipped you take minicrit damage
No randomn crits

I think a laser gun for electronic things is good

Pro
+20% more damage on buildings
+No ammo needed
Kontra
-20 damage on players
+10% longer reload time
And the ammo like the righteneous bison

Alt+Fire shoots a charge that sets enimies on fire

10% faster move speed
15% faster reload speed
-20% damage

does not require ammo

Pros
+15% Firing speed
+40% damage on building
+Projectiles will set Enemies on Fire
+Will absorb Metall of Enemy Engineers
Kontras
-40% Damage on Players
-Projectiles will not fly in a straight Line
-50 Metall
Neutrals
Dont need Ammo
Shoots Lazzzzzeeeers :D
No random Crits

I think that wep would be AAAAWWWWEEEEESSSSSOOOOOMMMMEEEEEE :D

And has 5 Shoots

I think a more of a rescue ranger style

Pros:
shots can remove sappers
shots deal extra damage to enemy buildings
Cons:
can be reflected
marked for death once a sapper has been removed
random crits are replaced with random mini crits

I dislike the idea of this being a lazer weapon, because the Engieneer already has llazer weapons.

One it has a bullet ejection chamber. It was desinged to shoot bullets.

Secondly the idea was to base it around gaining minicrits from the amount of times you get someone to use your teleporter.

25% slower firing speed
- 4 bullets in a clip

+Damages sappers
- -%30 damage

+Shoots polonium rounds. Applies radiation poisoning on hit- 30 dmg over 5 seconds. Target will glow slightly. Also decrease healing target will recieve by 25% for 5 seconds.
-95% max ammo (polonium is ♥♥♥♥♥♥♥ expensive!)
-2 bullets in clip
+50% greater weapon spread. (polonium decrease aerodynamic properties of bullet)

Fires nails instead of bullets?
+25% Fire Speed
+50% Damage on players
-50% Reload Speed
-25% Clip Size.
-25% Weapon Draw speed

Engi is a support-type class (even though it's defense), so how about this:
+50% Accuracy
+10% Reload Speed
-30% Damage on players/bots (mvm)
-30% Firing speed
-25% Clip Size
-40% Max ammo
-100% Damage on buildings
Depletes Ubercharge charge, destroys banners deployed by the Soldier, lowers the Demo's headcount when using an appropriate weapon, and negates other misc effects upon a hit.

How about as a "laser" weapon for the scout?
+Builds [Energy?] as you deal damage
+When the meter gets full you can either attempt to triple jump to get the sodapopper (?)
+ or when you AltFire with the pistol it can give you the Bonk Atomic Punch affect (?)
- 10% lower jump height
- 15% Vulnerability to Bullets (Other scouts become far more menacing)