Well, James... your post is rather vague in what you need to accomplish. Simple tips for writing or changing a rules set is starting with what you know and injecting what you want. Balancing is the hard thing to do-- but ultimately, the path towards balance is to write what you want, and then playtest it. I am a professional game designer-- and I have been sort of de-rezzing D20 into a lite-hybrid system for a new group. First, I redid the character sheet... the systems revolve around the character record, so start there. Second, I took huge swaths of d20 srd material, copy/pasted it into my private google sites repository and simply edited it down and rewrote the things I wanted to change. When it came to working out simpler character classes and such, I broke it down into a point system and worked out equivocating values for various attributes and scores that could be leveled... This allowed me to give the player options for upgrading the character while not making one option seemingly overpowering vs another. I don't really know how to give you tailored advice on such a broad scope question. EDIT: I'll also say that I wouldn't worry too much about breaking the game-- Alex is right, you are attempting to improve something that has already been put through the balancing paces. But if you are unhappy with it, and you don't mind ad hoccing the balance as you encounter problems, then break away!
