Yeah, I'm definitely planning on copying some of the planes from Book of the Planes, since it was released under the OGL. I've recently acquired a copy of the rare book "Gary Gygax's Cosmos Builder" and I'll be using it for ideas.
So what I want to do with the outer planes is have one for each of the different kinds of afterlives and pantheons in mythology. The Happy Hunting Grounds for the Plains Indians, Valhalla and Hel for the Norse, Limbo/Inferno and Purgatory and Paradise for Christianity, the millions of Hells of Chinese mythology, Asphodel Meadows and Elysium and Hades for the Greeks, etc. Plus other kinds of outer planes, nexus planes and planes-spanning structures like Yggdrasil and Jormungand.