Quote Originally Posted by the_david View Post
What I came up with was a solar system (technically not a planet.) with the celestial bodies as the elemental planes.

The sun would be the plane of fire, with the planes of air (Gas giant), earth (Solid planet) and water (Liquid planet)

Easy enough right? Well, until you have to start figuring out all the orbital mechanics and the other stuff.
Things to consider:
All of the planets would be pretty inhospitable if they where made up purely of their elements. All the planes would need an atmosphere. Gravity is another one, I'd prefer to have gravity equal to Earth on all planes, but that would result in really wonky orbital mechanics. How big would each of the planes be if that was the case?
The liquid planet would benefit from polar ice caps. That means it has an axial tilt, which doesn't seem right for a ball of liquid. It could be tidally locked though, leaving one side as a massive ice cap. Big enough to have some adventures on.
The solid planet would need a body of water to sustain lifeforms on the surface. It would be more like Earth and less like Mars. It might even need a molten core if you want to do vulcanoes, and use all the other benefits the Earths molten core gives to us.
The sun would need oxygen if you'd want to put a City of Brass there, otherwise it would be just a hot ball of plasma.

Okay you got me. I don't know enough about solar systems to make this plausible.
That's actually how they handled it in the old 2nd edition setting Spelljammer, which is basically "D&D in space". Well, they didn't tie the actual elemental planes to specific planets, but the main characteristic of each planet was the predominant element - earth, air, fire or water. Fire "planets" were usually suns, and "air" planets were pretty close to gas giants. Earth planets were regular, Earth-like (as in Terra) ones, and water planets were fictional liquid planets close to what you're describing. Each major campaign setting was given a solar system, which didn't necessarily conform to modern physics, since Spelljammer worked under a paradigm of "fantasy astrophysics" whose laws are quite different from the real world but close to mythical conceptions of cosmology. For example, some systems had the campaign setting's planet at the center and the sun orbiting it.