I've always thought that many of the races in 40K take their roots in other Sci-Fi sources. In these conversions, I take that idea a little further. The Aliens are Gaunts with 'stealer heads and claws and the Predators are Kroot with lots of green stuff.
I have converted a few random figs over the few years.
Link from Zelda, Ironman, Storm Trooper from Star Wars, and Medieval Batman (the Dark Knight):