One of the things that D&D does well, and that a game like, say, GURPS does poorly is sufficiently staging the complexity for players. GURPS, has to a lesser extent remedied this by releasing GURPS Lite, and they smartly made it free. In that vein we’re going to want to start with a set of minimal rules that can be expanded upon without violating those minimum rules. This also lets us spin up a game we can start testing sooner, and makes future rules modular.
So what do we have to do to get to Living Myth Lite?