Multi-genre
Chaosium's Basic Roleplaying (BRP) rules, released in 1980, are considered the first commercial generic roleplaying system. Worlds of Wonder added rules for multi-genre support to cover similar ground as the Hero System used in the 1981 superhero game Champions and later products released by Hero Games. The idea, as stated in the pamphlet, was that characters could be created in any of the three settings, or even Basic Role-Playing, then physically travel to any of the three adventuring settings. The pamphlet proposed a neutral area that had magic connections to each of the worlds. Money from each setting would be valuable in the crossroads, at various conversion rates, and characters would be able to physically travel from one world to another.
Each setting booklet, however, was effectively a game in itself, with different (though mostly compatible) character generation and rules.
Read more about this topic: Worlds Of Wonder (game)