ersatzphil
Hero
So, what's the subject going to be? Good and Evil? Powerful characters/NPCs/monsters? Magic and spells? Heck, could it be a general Greyhawk or Planescape setting book?
Given that Iggwilv had mansions in the Abyss and the Grey Wastes, it actually really might be a planar-themed players option book. I was personally campaigning for "Shemeska's Guide to the Wheel", but I'll take what I can get.