I noticed in d3d you can always set Z=0 (except for camera) and just use x and y for a 2d in 3d game, use textured squares or whatever. Then you can zoom in and out by moving the camera back and forth. Anyways make a bunch of planets, with outer layers having low density and inner areas high density, then the density gives you better items to craft. Maybe throw in some plants, animals, trees, water, gravity, cave-ins, and that sort of thing, along with a way to make space ships with armor. Larger planets are hotter and denser in the middle.

I noticed in d3d you can always set Z=0 (except for camera) and just use x and y for a 2d in 3d game, use textured squares or whatever. Then you can zoom in and out by moving the camera back and forth. Anyways make a bunch of planets, with outer layers having low density and inner areas high density, then the density gives you better items to craft. Maybe throw in some plants, animals, trees, water, gravity, cave-ins, and that sort of thing, along with a way to make space ships with armor. Larger planets are hotter and denser in the middle.

If you have a planet with multiple layers, but it's in 2d, will it be a side-scrolling game? The thread title made me think immediately of Clonk Planet, which is a good thing, but I'm not sure I'm getting a good grasp on your vision.