Because why not, seriously. This is mostly because I wanted to see what people thought of some ideas, but I'll probably add more in, and you could even add your own in. Just try and keep them balanced.

"Twin Peaks" - [Engineer shotgun]
Can fire two shots either quickly or at once
40%+ pump time between the two shots
-20% damage per pellet

"Dual Glocks" - [Sniper smg replacement]
40%+ more damage per bullet
Accuracy cooldown.
58% clip size
-40% fire speed
-60% damage in melee range
-50% reload time
(this one was mostly for the hell of it, and because I use sniper for medium range combat half the time.)

"Burst Syringe Gun" - [Medic Syringe gun]
Fires four syringes at once
-90% clip size
-80% fire speed
-50% total ammunition
(Each group of syringes counts as one "bullet"/"shot", would probably deal around 75 damage should the entire group hit)