Honestly, what I would really like to see are some books for the default setting that help you to make you own settings. Like the d20 modern postapocalyptic book.
I don't want them to make 4 mini settings in one book, frankly I don't like using any settings other than my own, I just want a big book of ideas, towns, Society structure rules, maybe some rules on different climates (jungle, desert, islands and sailing, etc), organizations, cults, guilds, that sort of stuff.
I just want to make my own settings with my own storylines... but if wotc had a book that could help make that easier I would definately buy it.