Title says it all, but here are some rules...
1. You would have to except the weapons flaws (Unless they make the game crash)
2. If you want it to shoot different projectiles and/or function in a different way just say it, unless it conflicts with Rule 1.
3. No Ponies, I don't want anymore double entry.
--NEW!--
You can now put vehicles in.
Same rules apply, and here is an example of the vehicle version...
Vehicle: Monster Bengs 190
Into: Any Driving game I have.
Specials: Just Cause 2: Has 4 Minigun turrets on top that shoot tank rounds, and in the trunk is a gigantic Invincible Explodo-Ball Launcher, Basically, it shoots a ball that explodes everytime it hits something, and can't be destroyed. But it can be deleted.
Gran Turismo 5: Has 1050 HorsePower and wieghs 2500 Kg, can be upgraded to 1350 HorsePower with 1750 Kg.
GTA4: Very rare, very strong, has 2 Tank turrets sideways and mounted to the sides of the car, and has a Lazor Jet Cannon, which has enough recoil to send the car at speeds of that of exotic cars. The laser can also do serious damage to tank over time, and could even Kamikaze ram helicopters by pointing them down.
I'll give an example of the weapons version:
Weapon: Uberbow
Into: Just Cause 2
Specials: Extremely Explosive Headshots