Hear me out: Arbalest and Bastion are kinetic special weapons with intrinsic advantages against Barrier and Unstoppable champions, respectively. I propose Bungie complete the trinity with an exotic kinetic trace rifle strong against Overload champions.
To make sense as a kinetic trace rifle, perhaps it fires an uninterrupted stream of magnetically-propelled needle-like projectiles? Something along those lines? In any case it seems to me that there's a practical gap to be filled here.