I wanted the gameplay to be unconventional as a result of the weapon, so I eventually settled on this idea:

Firing globs of liquid that freezes enemies on contact.  The enemies unfreeze after a few seconds, so the only way to kill them is to have frozen enemies collide with each other.  I haven’t decided if I want the player to remain stationary or not.

So far, I have enemies spawning, approaching, and freezing, but not colliding yet.

No screenshots for now because my programmer art is a bunch of rectangles. Hopefully, I will finish the core mechanics early enough to spend time on the art.