I've not really given much thought to how other planes would be dealt with in my "folkloric" concept of setting design. But I did have an insight this morning that I thought might be interesting to play around with.
What if the "other planes" were, in terms of their geography, shadows of our world? That is, the Fairy Realms would have mountains, rivers, oceans, etc. in roughly the same configuration as they are on Earth, but with differences in composition. A forest on Earth might be home to normal animals and such. But it would exist as a sort of "parallel universe" with a nearly identical forest in which dwelt satyrs, elves, and dryads. Unknowing travelers might slip from one dimension to the other without even realizing it, and thus we have encounters with the supernatural in what they think is the normal world.
Other worlds would exist similarly; Hell would have rivers of fire in place of normal rivers on our world, Heaven would have its gleaming cities in the same locales as they exist here, etc. Even political boundaries could be dimly echoed in the alternate worlds; where King Roderick rules a realm on Earth, so the fairy king Oberon might rule a parallel kingdom. (Another thought; what if there were a fairy counterpart to every person? Or a demonic and angelic one?) And, as one moved from one kingdom to another in the "real" world, and the legends and lore concerning the Unseen Folk (or demons, or angels, etc.) changed according to culture, so too would the nature of the other worlds change accordingly. What works against a demonic presence in one land might be completely useless against one a thousand miles away, although naturally the local sorcerers would know what was and was not effective.
Magics could be used to create portals between the worlds, charms and incantations used to contact beings in the other realms, etc. Heck, with a little massaging, this could possibly be turned into a coherent basis for the magical system itself.
Happy 50th, Star Trek - I was just a bit too young to remember watching Trek when it was in first-run on television (but I was alive then, and it's certainly possible that I was ...