It's a curious thing, stripping the setting out of a game book and putting a complete set of core rules in its place.
It's also curious having the freedom to invent my own mechanics, and finding that most of the time I'd rather just go with what I know.
Anyway—a complete RPG book needs, what: character creation, items & equipment, rules for dungeon-crawling, rules for wilderness adventures, rules for combat, maybe some notes on vehicles, and then spells, items, monsters, and some encounter tables?
And a lot of that stuff, even from basic D&D, I don't actually use. I just wing it, or I use a vastly simplified system (like what's found in Retro Phaze—a lot of that may just get lifted for E&E 2).
So with the first chapter done, I have character classes and equipment all hashed out, which puts me at about 20% right now. I wonder if I'll have this done by the end of summer.