Monsters need front and back spacing distance settings between them. Because it's a flat image, melee enemies look a bit strange when stacked in front of the protagonist

Enemies are missing a setting for whether or not they can be blown away by the thrust of the weapon when they are killed, if the enemy is a stationary machine gun, a tank, usually they shouldn't be able to be knocked away by a shotgun

Wish monsters could attack more often, which makes it possible to create a machine gunner enemy

Hopefully there's a place to add more explosion looks, there's only 1 at the moment so I can't add green explosions like insect acid explosions or plasma explosions

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Some other things that could be improved but are not important

Weapons lack the ability to eject shells and the sound of ejecting shells.

Weapons lack a setting for where on the screen the projectile will be fired from, it's very common in FPS to have the gun in the bottom right corner of the screen.