These first three are more minor quality of life changes, just things I found a bit annoying but aren't a massive deal.
First, aiming while crouching. Instantly standing up when you aim is annoying at times, especially against the turret grubs. If you press it on accident then you can take some stupid damage...
Second, health upgrades should give you a heart. Maybe it's just because I'm too used to it after The Binding of Isaac, but I think it's a little annoying not getting that extra health. It's doomed me a couple times because I got a health upgrade instead of a normal heart when I only had half a heart left.
Third, the missile should be able to be moved from the second slot. I'd rather my ammo-less weapons be first so I don't accidentally waste missile ammo firing
My major suggestions are this:
Make the spark generator a modifier. I think I mentioned this on reddit but I'd like to put it on the official forums. I think the Spark generator would work better as an electric modifier similar to the flame modifier. Bullets could still work exactly the same, firing in that erratic pattern and causing a decent amount of knockback. I feel like it'd allow for some really cool new combinations, like an Ionic flamethrower that electrifies enemies and makes them bounce off of each other when they touch or a laser that fires out like an arc of electricity or something
Maybe make the laser a constant fire weapon like the flamethrower and saw? I'm not sure about this because it might make it a bit overpowered and it'd also make the pea shooter the only weapon that isn't automatic in the game. But it'd make sense, since lasers stay on until you turn them off in real life.
A modifier that increases the size of bullets could be cool and add some variety.
Finally, I'd like to talk about boss items and the weapon slot upgrade. I personally feel like the weapon slot upgrade should be an item you can find randomly, and that the boss should drop a random upgrade. I think this gives you the opportunity to find more upgrades and do more with world 2. Maybe not now, since area 2 isn't really done, but definitely in the future. Having the weapon slot upgrade show up as a normal item in shops, the optional item room, or around the map rarely would also add variety and change up the game a bit.
Also for a potential future area, maybe a water theme?
Those are my suggestions for now, and so far I'm still having a ton of fun with this game. I keep finding new rooms and new interesting patterns, and this game never fails to be a fun way to kill time.