I called it the Any-System Key. It fit it on two pages, one for the basic concepts and the other a kind of worksheet for listing relevant skills and translating difficulty numbers (a third page added later offered some sample stats across genres). The system focused on using basic skill descriptions and three levels of adversaries (henchmen, boss, and mastermind) to give gamemasters some guidance adapting these notes to their preferred game system. The Any-System Key worked fine for the game material I was writing at the time, primarily pulp content like Heroes of Rura-Tonga and Pulp Egypt, using game engines like the D6 System that relied heavily on skills and difficulty levels to define play parameters.
But now I’m considering designing some supplements in the medieval fantasy genre, something more compatible with the class/level system of Dungeons & Dragons and the numerous retro-clones made popular over the last few years by the Old School Renaissance movement (OSR).
Like the Any-System Key, the OSR version seeks to describe characters, creatures, and other difficulties (such as those corresponding to saving throws, thief skills, and turning undead for clerics) in broad terms, providing enough material so gamemasters could port such setting concepts into their preferred OSR-style game engine.