I like the idea of an open combat system, but I realize that the more flexible the system is, the harder it can be to code.
Since 3E, WotC has been pusing the d20 idea. Basically the idea is that the core rules are general enough to be used in all types of games from Star Wars to Call of Cthulu. A lot of other game publishers are licensing d20 and converting their systems over. IMHO, this may not be the greatest thing for the gaming community, but d20 does seem to work fine for D&D (I've played d20 Star Wars and I didn't think it was quite right).
Anyway, off the WotC site you can find info on
d20. I'd also check out the
Open Gaming Foundation which has most of the OGF rules posted in RTF (looks like a vanilla Players Handbook/DMs Guide, etc.).
As far as I can tell, you are free to use the OGF licensed rules in your game (you just can't use WotC trademarks).
Sil