You will be able to get and add mods to your weapons. Some weapons have mods that make them do very different things. Example, a hammer mod lets you smack the ground and create a shockwave that travels along hurting enemies.
It is unsure, but likely that there will be a flashlight mod that allows your weapon to shine some light in the direction it is pointing.
The rarity of a weapon determines which Weapon Mods may be included with the weapon. This table (incomplete) shows what the different Rarity values unlock:
|??||explosive shots, bouncing bullets, branching shots|