Currently, the only 'instant' attacks in this game are lasers from various enemies and bosses. Given that a laser-like weapon for the player has been discussed to death, I wanted to propose something a bit different: A gun-like instant-travel hitscan weapon that deals all of its damage in a single frame.
Railgun currently *almost* fills that niche (aside from obviously not being hitscan), so having it be rapid-fire would potentially make it more unique. Some sort of built-in siege weaponry effect would also help distinguish it from other weapons.
I also imagine it having some sort of inaccuracy, being shot at a slight angle based off of weapon spread. Homing mods could potentially cause shots that would just miss enemies to 'snap' onto them instead, and projectile velocity could maybe simply act as a direct damage boost.