Overall, the game is fun for the most part and surprisingly polished. You've got a proper settings menu with features like customizable keybinds, resolution options, and other small quality of life settings, which is always appreciated. There are also nice gameplay touches like screen shake, particle effects, and even a skill upgrade system! That said, there are some things worth mentioning:
UI:
• The upgrade menu pops up unpredictably from the player's perspective. Maybe a UI change to make it clearer when you'll reach a new level, or perhaps sound effects signifying XP gains with variations based on how close you are to leveling up. Either way, it needs to be communicated better to avoid surprises.
• The default accept keybind (ENTER) disrupts gameplay when you reach a new level since you can't reach the key with your left hand, so you end up having to stop holding the mouse or use the other hand that controls movement (WASD) to choose the new upgrade, which sometimes causes you to take damage unfairly. I know you can change the keybind in the settings, but it's best to avoid the problem by default. A suggestion could be making both SPACE and ENTER the default accept keys.
• A small change to the slider, it's more convenient to not have to press the accept key before modifying it. Simply moving right or left would feel cleaner.
• It might be a good idea to have the difficulty setting pop up when starting a run. You could also take the opportunity to inform the player that it can always be modified in the main menu. If it ends up feeling too repetitive, then make it only appear on the first run.
• It would be nice to see what upgrades you currently have, for example displaying it while choosing a new one, so you can make a more informed decision, or also displaying it when pausing, or maybe pressing a button to open a menu, or just always displaying it in the bottom corner or something.
Game Feedback:
• Sometimes it's unclear or hard to notice when the player takes damage. It needs to be communicated better, maybe by, for example, using the same hit effect enemies have, doing a fullscreen effect like briefly overlaying the screen red or black, or using some screen shake.
• The burning bullet upgrade lacks visual feedback, like a different bullet sprite or a fire particle effect on the enemy. Another thing is that it can be a little too OP, making it especially easier to kill those small enemies that are normally hard to deal with.
• Vampirism upgrade feels a bit too weak. 0.2 per enemy feels like it's a little too low since you usually don't kill that many at the same time. It also lacks visual or sound feedback to inform the player that it's doing it's thing.
• Sounds are sadly a little neglected. For example, it's missing sounds for the player shooting, bullets hitting, the player and enemies taking damage, burning sounds from the burning bullet, hawk sounds, splash sounds for walking on mud, and footsteps. An important thing about footsteps is that they can't sound repetitive since you'll be hearing them 95% of the time.
Balancing:
• Those small fast enemies are difficult to deal with, especially if there are multiple of them. This could be improved by adjusting their speed, giving them a slightly bigger hitbox, or maybe making them get knocked back after hitting the player so there's some breathing room.
• A short damage cooldown where you can't be hit again immediately would help a lot with balancing the game and keeping it polished.
• Enemies being affected by the mud, like how the player is slowed down, makes the game more dynamic and adds some strategic play as an option. It also helps a little with keeping enemies from always clumping together.
• An easy way to add variation to the enemies is to make their movement speed randomly vary slightly within a range and make each enemy type have a different base speed. (also helps with the enemy clumping problem.)
• It's hard to make the hawk attack enemies. Some target assist or something would be nice.
• The intro is nice but a bit repetitive. It might help to shorten it or change it slightly after the first run.
• The player is not under a lot of threat when it comes to projectiles; there's only one enemy type that can shoot, and it's relatively easy to dodge the bullets.
New Stuff:
• New weapons for the player, maybe melee ones too.
• New enemy types.
• More upgrade options, maybe even levels of them, with the upgrade getting strong every time.
• An option to toggle VSync, and another to cap FPS at a specific value or leave it uncapped. (For high refresh rate monitors)
• Adding sprites for the enemies with the same art style as the player.
• Walls, though the enemy AI needs to be taken into consideration, but it's a nice way to take cover or to strategically separate enemies into smaller chunks.
• The level is relatively empty, with only mud inside it. Simple visual elements like bushes or trees can make it look a lot better.
• Variation in level design, a few manually made ones, or randomly generated simple things like random amount and position of muds (and walls, trees, bushes, etc.).