A gun that consumes health upon fire and the more you continuously fire it the more health you waste
would be very cool. Of course it would have to be able to do a fair amount of damage since you are sacrificing health for it.

A grappling hook gun which could be used to fire upon an enemy and promptly drag them toward you, this should not
do damage but rather be used for dragging enemies toward your team so that they can swarm them and get the kill.