Wayfarer Tactics is a free wargames rules designed to be universal in both setting and scope. The authors write:
Welcome to Wayfarer Tactics, the dynamic miniature wargaming system that puts the whole of the Multiverse under your control!
Using the simple dice mechanic of Pocket-Tactics, you may enact epic battles using whatever models you have on hand (in any scale). You’ll also need a deck of standard playing cards to give your characters orders and activate certain special abilities. Whether you want to play free-range skirmishes in dense terrain, massive squadron battles measured by the yard, grid-based dungeon crawls, or anything else, the modular core rules will adapt to your needs. It can even be played as an RPG!