What you have is pretty good. The series does shows ambition, and the variety is nice, but there are a few things you could improve. These sprites are too dark to be Doom style. Many of the colors you used are overly saturated. The laser red being the most apparent. Also, your selout also seems too dark. This creates the appearance of a solid dark colored outline. You might also notice that in the Doom sprites there is no pure white, and only two pixels are actually black. I'm not even sure if those two pixels are pure black. Color reduction would probably do some good as well. Try leaving more space between colors. You don't need seemless blends for every shade; in fact, doing so creates a gradient effect that is not very nice. I also have a nitpick. The background color burns my eyes. Once again, good work so far.