Here's my honest review: I think the project is not all finished (I don't know what happens with your team). In the next update, you have to add a sound that indicates where the zombie is and a sound when the player gets hit. Sometimes the zombies spawn behind me, and I don't know what's going on. Then I just go into the nanobot phase.
About the gun. I just don't think the pixel particle effect when you shoot is exactly what you need in this game due to the art style. You need the real one that matches the graphic. If you want to keep this, I think a graphic like Doom is exactly what you guys should choose.
About the nanobot. I think a nanobot moves around our blood vessels, and ours are not prickly thorns. That's a little misconceived for me. I don't know; if you want to make it a little challenging, you can make it avoid blood cells.
And yeah, you may consider Doom's art style because your art style (for now) just doesn't fit (I don't know other people's opinions, but at least for me). Good luck, guys!