I was looking at the various pantheons in the Dungeon Master’s Guide, and while they don’t expand on the personalities, etc. I’m leaning toward using either the Dawn War and Forgotten Realms pantheons (as competing religious traditions in different areas), as well as the nonhuman deities from the Players’ Handbook.
Are these gods “real”? Maybe — as gods by ancient standards weren’t omniscient or omnipotent, but powerful enough to impact the world in grand ways. I’m leaning toward a Hindu-like division of devi and asura. They’re both “gods”, but one is divine and other malign. This works well with the traditional fantasy tropes of good and evil, but also fits the alignment conceit of the rules. Rather than a necessary behavioral guideline, it’s more a tie to which side of the grand fight you’re on. Demigods and demons are the foot soldiers of their mythic creatures.
I do know I want to lose the connection to Earthly pantheons and notions of divinity. There’s too much baggage with that, and I’m doing Hindu and classical riffs for our Hollow Earth Expedition game, and Battlestar Galactica was completely infused with Greek myth (which I love.)
Connected to that are the “planes” and the idea that they house these creatures. The dozens of planes aren’t needed. Astral and etherial are more as a function of their connection to spell-users. The Shadowfell and Feywild are much more useful — strange worlds just a half twist off of our own. (I’m seeing the Upside Down from Stranger Things for the Shadowfell, and a MAxfield Parish-like realm for the Feywild.) Then you need the positive and negative energies, or planes. It’s not Hell or Heaven, but rather Life and Death. But are they really dualistic? That could be part of the mystery for the wizards or magic users — what’s the real nature of the universe?