I wasn't really feeling too well about the theme until I decided to not make a video game and make a tabletop instead. Now I'm making a card game with draft and trading elements. Thematically, it's about earth elementals fighting a civil war. In order to fund this war, they sell the different minerals they have in their body to buy weapons and whatnot. Their defenses are tied to how strong the minerals are in the different limbs of their body. Of course, different minerals have different monetary value to merchants (gold is more valuable than quartz).

After about 3 hours of brainstorming and spitballing ideas with a friend, I have a somewhat fleshed out design doc. A couple more hours and now I've got a preliminary rule book.