Just a minor nitpick, and probably relatively easy to implement, but here goes: When enemy AI engages you, it always shoots at the same frequency, which sounds *very* mechanical (bam-bam-bam-bam-reload-bam-bam-bam-bam, hope you get what I mean).

The idea is to introduce minor random variation into the shooting, like, add a random delay of up to 100 milliseconds between shots. That way, the shooting sounds much more real and less robotic.