At a quick glance, I found the combat choices really interesting. I'm curious to see how they'll work in practice.
One thing about the license — if the idea is to encourage hacks and supplements, maybe releasing the text under a CC‑BY or CC‑BY‑SA license would be more interesting. Then you can keep the images and layout under closed Copyright if you prefer.
Anyway, great work. I'll take my time to read it properly and give feedback on the other parts.