This is pretty cool. Haven't read in detail yet, but I was wondering if you could slot in some Empyreal Realms? You've got the Underworld and they seem like they would be the accessible parts of the heavens. I'm thinking Dante's works, Greek Elysium, places where mortals have had heavenly encounters. Probably need to be accessed on holy mountains, that kind of thing. Harry himself thinks Heaven must be somewhere in the Nevernever.