Struggled with the theme for the last hour but I have an idea that I like. Here it is:

Having recently become a huge fan of the art tradition of minimalism, you’ve decided to make the entire world minimalistic. You do this with a special gun, the MINIMALIZER. It can minimalize color, or minimalize health (like a normal gun, I suppose), or minimalize movement, or what have you. But of course evil enemies are standing in your way! Go show them that minimalism is actually a good theme after all!