A tabletop RPG designer named Michael Prescott proposed that people design stand-alone RPG subsystems that groups could mix and match together to create their own custom ruleset for play. To me that means the systems can have an effect on the shared story, and can be triggered for use by things that happen in the shared story, but can't otherwise interact mechanically with each other. So, like, one subsystem for character creation can't tell you to create a character with hit points that another subsystem for poisons tells you to reduce. I think it's a super fun design space. Theoretically the ones I've made can be mixed and matched with ones by other designers.