A book that actually was all flavor that gave a specific rationale to each of the rules would be nice. Perhaps a generic gameworld with an explanation (or several possible ones) for every one of those crunch rules. You know, a flavor/fluff book.
I would like the 8 schools of magic and 9 spell levels to have some mystery in my campaign world. I know that prestige classes are supposed to be placed specifically regarding culture and history, so why not all the feats and specific uses of skills as well?
The freedom to be any class matched with any race, or place any monster in whatever part of the world does mean more freedom. But shaving off all of the serial numbers from the rules (sometimes in terms of why a rule was created in the first place) means no implied world history. And to me, that means the DM must make the connection between the rules and the background of the world. As most fantasy stories tend to be world discovery, not self discovery, I think this connection is very important to the genre. So a book that focused on creative means of doing this would be welcome.
/rant off