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I had a few more thoughts after posting:

I ended up with quite a lot of ammo as I got further in. Like 80ish pistol shots and 60ish laser shots, 12ish grenades before they got reset for the boss. Maybe it could be interesting to have some hazard that's hard to avoid and reduces your ammo based on how much ammo you have. e.g.  Tesla coil: if you damage this with a weapon, lose all ammo on that weapon.
Though, that kinda encourages people to leave ammo and pick it up later, which I don't think would be fun. Hm. Maybe it mostly appears on a specific map that has an anti-backtracking feature. E.g. a barricaded map where there's no ammo pickups lying around before the end, and all crates and stuff specifically spawn in 1-wide hallways so you can't walk around their pickups. That might be a bit much to add for just this idea, though.

Creating occasional situations where the player has to engage enemies from multiple directions rather than baiting from a hallway

A very simple idea is just. Hallways are sometimes 2 spaces wide. Might be hard to program for the generation, though.

Level with an anti-backtracking enemy??? An enemy that spawns in the same room as you, can't be killed, but is stunned for several turns when you shoot it. Might work together with tesla coil.

Mimic explosive creates? Look like explosive crates, but when the player goes to the next room after them, they grow legs and start following after a few turns. They still function like explosive crates, before and after they start moving.

Animations
The pistol feels feels much less snappy than everything else, particularly when you use it from a distance. I got used to it, but it stood out at the beginning.
Revealing a room animation is slightly delayed, if feels like players are supposed to walk 1 step further than they have to, then eventually figure out they don't need to. If it's intentional, that's fine.
Player damage animation is maybe a little too easy to miss.

Played one more run to be sure of these feelings. 
I experienced another off-screen explosion, but this one didn't kill me and happened when I was a few steps away from the hallway to that room.

Bug: I got a water level where the key is unavailable. There's just a closed door. Not sure if I actually moved up to the door or not. But yeah I didn't notice any key even though I killed everything. And the mushroom exit was really close to the entrance.

Bug: if you're standing in the area a spike can move into, and you shoot, you hit something in your own position?? Which is probably the spike's hitbox which doesn't retract with its hurtbox?

Bug: floaty boss was shown behind an explosive crate he'd dropped. (during phase 1)

Triggered a few eggs and let them hatch. Didn't let the spawned enemy get close to me, though. I like that they can trigger both from damage and walking close.

Didn't encounter any witches or flamers.

Ended up dying to second phase of the boss because I was playing too fast and lost count of how long it was since the last attack. But it was looking easy up until then, that's why I was playing fast. 

Potential bug: I feel like I saw the boss spawning 2 troopers in the same corner and they ended up inside each other. May have been part of why it felt easy, they never really came near me.

Maybe the boss would be more interesting with a smaller room, so the troopers spawn closer to the player.