Hey, very cool game! Took me back to high school when I was playing Crimsonland during IT class. :)
I think it's a very cool idea to incorporate the blood types the way you did and a fresh take on power progression! One actually has to decide which enemies to kill in order to maximise progression and prevent regression, instead of simply picking up or ignoring random power-ups that drop from random mobs or bosses, or choosing from a set of however many either by levelling up or entering the next room. I love that luck plays such a big part as well: first you have to get lucky with the power-up combination, and then also with the correct zombies spawning! And between those two random rolls, that's where skill comes into play.
I think you limited the amount of zombies that can spawn, I guess divided into waves, right? It gave me the opportunity to run after a specific combo I wanted for 2-3 minutes before progressing into the next stage, and I feel like you shouldn't be so nice, haha. You should force us to make tough choices by keeping the onslaught coming, and if we can't get that sweet ricochet or +1, or we're desperately trying to avoid the longer reload time, well, it's either that or getting overwhelmed!
But then again, my max time was like 5 minutes because I need to git gud, so maybe some mercy does not go amiss for players such as myself. :D
You could also expand on the shadow mechanic - certain mobs that spread shadow, others that spread light, or mix it into the blood types somehow so you gain some agency on how much shadow or light there is on the terrain. And again, make it so that you have to choose what's more important - more shadow, or a nice juicy gun power-up. Or make it so certain power-ups come as a double-edged sword. But these are just random thoughts of mine for where you could take it further, if you were planning on doing so - I had a lot of fun playing it already!
Very cool! Thanks.