I love the old school shooter vibe, it's fun, plays really well and I like the idea of having finger guns. The sound effects and the music also got that cool retro shooter flair going on.
For suggestions, I think the level design could use a little bit more detail - like little things that make you stop from progressing for a couple of seconds and just admiring the level details.
For music, I think something with a little bit more bang would fit the aestetics and general game design. Either some old 8-bit techno (like the original doom song) or even go with some drum&bass type beats.
But that's just some ideas, really loving this project, good work guys! Keep it up :) <3
Edit: For the general design I could see a game like this playing in a cyberpunkish world as well! :)