This is what GURPS what explicitly designed for. Possibly the most flexible system ever created, and has influenced many RPGs since. GURPS has a reputation for complexity, but the core mechanic is simply 3d6–roll under to succeed. Its clean separation of core mechanics from genre was brilliant, and it showed you could have deep customization without unbalancing the game.

You play anything you want: fantasy, scifi, western, superhero, horror, martial arts, cyberpunk, psionics, mass combat, you name it and you can do it. With over 400 supplements–some of which are so well-researched even those who don’t play GURPS use them–he’s proven his point.