I would love to hear some opinions!
And I’m still working on it
I mean the first thing I would work on is the HUD (heads-up display), which consists of the healthbar & other icons. Right now the healthbar is somewhat difficult to see since it’s transparent and it appears that certain parts of the stage cover it (like the rocks and the leaves in the second area); you could add a border and a solid background to the healthbar and the other icons to help them stand out.
What I recommend is to put all of the HUD on another CanvasLayer so it’s simply drawn above everything else, unless you have a particular reason not to. Using different CanvasLayers can be very helpful in situations like these
Bullet trails and better contrast between the background and things the player can actually collide/interact with
If you are just fishing for ideas, maybe add jumping, enemies that stay at a certain height that you can only shoot while crouched for low enemies, jumping for higher enemies, or standing for normal height enemies. That could add some variety to it.