I wrote a ttrpg called 5E. It is, undeniably, the world's best fantasy roleplaying game. If you are interested in creating third party content for it under my generous licensing terms, I'm running an itch jam here: https://itch.io/jam/3rd-party-content-for-5e

Thanks for making the 5 E. It is a very inspiring product. I am totally in your Jam....