It's probably smarter to have a modular ruleset. Have some very basic rules which can be modified for each game. Get too many rules, and you limit the genre of RP.
Also look into some non-d20 systems like
GURPS,
The Window and
FATE. d20 has a lot of shortcomings, like the flat die curve, gaming the system, and extreme XP-hunting/leveling-up (which works for the video-game paradigm, where you become more and more powerful, but not for a more limited and gritty paradigm where you have a cap on your capabilities and don't advance to become a demigod).