The other [product I would love to develop] would be the book I keep suggesting that WotC consider publishing: Core Book IV: Game Designer's Guide. The GDG would contain all the systems and mathematical analysis used to construct 3e, and explain how the system works at a low level, and how to add to it and extend it correctly. That book would be a guide to effective encounter, scenario, story and world design, all using the d20 System. It would require tremendous R&D work to do things like figure out a quantifiable CR/EL system, among other monumental tasks. The great thing about that book is that not only would it be a boon companion to the 80+% of DMs who write their own content, but it would be a huge step up for all the OGL/d20 designers who are working from the black box of the DMG.