I'd like to keep the pieces I build compatible with the 5e SRD. If you look at the Challenge Point system I'm working on as a CR replacement, it's designed to function with any creature and give you a baseline point value based on CR.
The idea is to build modular systems that complement and improve what you have, rather than make something that forces you to pick A or B.
If I did a full game, it would be tied to a new world with its own set of ancestries, classes, magic system, monsters, and so on. I have some ideas for fantasy and science fiction worlds, but it is super early in the process.