I narrowed my 20 thumbnails down to 5 ideas and refined these ideas a bit more.
I decided that each color of minion in my game would be a sort of "suit" and have its own theme. So the black minions are sort of hellion, ghostly types, the blue are robotic types, the purple are alien, the yellow are toxic waste mutations, and the green are "monster" or beastly types. All of the types of minions are represented in the sketch below.
These last 3 feature ideas I came up with for the characters that players could choose from in the game, and each villain ties into one of the 5 minion classes.