"Hey! A Mystery room has appeared! Find it before it DISappears!"
Stupid suggestion (prob april fools special in pre release 3-4):
Double Barrel Shotgun, a DooM 2 reference.
If to think, a new hud element, with tracer hitbox, and everyone who get hit stopping to exist (no literally, they just disappear for censorship-less apperance).
Can be found in mystery room (if you'll make them, probably in generated floors, but make them not bb+ exclusive please. Oh, and alternative could be RNG).
Do it in your free time, and don't focus on this stupid idea of mine right now. Have a nice day, Copybasics!