Okay I just feel like being creative so I'm gonna throw some ideas out there.
The Germ Spore gun
How does it work? Think of the can of bug spray in any rats map. The gun will shoot a sticky, maybe even hardly noticable, substance onto a wall, anyone who comes into a specific distance of it will systematically be drained of life until the person dies or the affect wears off. Could be good for warding off flanking enemies. Any modifications are welcomed.