Yeah, the AK is supposed to be one of the "main" weapons, because the others, while powerful, would be OP if they were much faster/stronger/etc. The shotgun is a bit disappointing, but it has the highest knockback other than grenades.
I'll take a look at the Flamethrower, because based on feedback, that's the weapon people have the most issues with. I recently doubled its range, so that should help.
All of the artwork is from the original game and will change before the final release. My girlfriend is working on the concept art so I actually have some art direction to work with on the pixel art as opposed to just randomly throwing shit together until it looks nice. As far as the "fire directly from your ammo stores" powerup goes... that's the unlimited ammo powerup. You don't actually consume any ammo while that's active. As far as stats go... you get 10% extra speed or damage from each pip in those stats, but 20 health from each pip in health.
The sound effects are terrible. I'm aware. I'm really just a software engineer. I don't really have much artistic or musical talent. The soundtrack is a placeholder. Since I don't have much musical talent myself, I was thinking of coming up with a genetic algorithm to "evolve" new music using other synthwave songs as samples (the end goal for the game's art direction is 80s style with synthwave music).
As for enemy damage variety, I'm currently upgrading enemy and weapon damage and player and enemy health to a d20-based system, so they will have a damage range as opposed to a flat amount. I'm also going to change their attacks so it's not just "oh, he's touching me, so I take constant damage", and it will use an animation to actually show them attacking, and you'll take damage when they hit you.
The spawn fountain idea is interesting... that's sort of the direction I went with to invent "Big Mama", the one that explodes into a cloud of smaller zombies. The idea is that if you don't kill her fast enough, you'll have to contend with a large swarm. In later waves, when there are more than one, it can get pretty crazy.
Thanks for the feedback!