Zardnaar
Legend
In a setting book, rather than actual adventures, I think lore reasons/opportunities for high-level adventures would be easier to write and more useful at the same time. An author who's never heard of you can't predict the specific crazy abilities of your party, but they can go "a lich in Daggerdale is replacing farmers with his shapechanged dragon slaves to monopolize wheat production, Elminster needs a group of 18th-level flunkies to sort it out for him before his scones start getting more expensive." Add in some local flavor and some setting details, and let the PCs figure out how to solve the problem.
You've read my DM notes then!!!