@Tim_Eagon #CortexPrime, and maybe #SavageWorlds. In the past I would have said #BRP, but that is now available; also #ArsMagica has announced they will have an open licence soon.
Most of the good ones already have CC licence, e.g. forged in the dark, powered by the apocalypse, fate, gumshoe, d&d, etc. https://sgryphon.gamertheory.net/2022/06/open-source-alternatives-to-dungeons-dragons/