Right now, I'm in the process of building a custom set of Elemental planes, based on the intersection of the four Elements (Earth, Air, Fire, and Water) with the five Elemental Energies (Fire, Cold, Acid, Electricity, and Sound).
The Elements and Elemental Energies each have their own planes, and then there are a bunch of demiplanes scattered around between them. (Some stolen from MotP or Planescape, some original) I've got demiplanes of Ash, Cloud, Dust, Fog, Frost, Glass, Magma, Mineral, Mist, Ooze, Radiance, Salt, Sand, Smoke, Sparks, Steam, Vacuum, Vapor, Wood, and anything else I come up with before I'm done.
After I do that, I'm planning on designing various demiplanes for the Deities in my world to inhabit.