Just some quick answers...

Gravity, atmosphere, etc.: I think my aim, initially, at least, will be to make Earth-like worlds and possibly also exaggerated sci-fi types of worlds based on climate zones like desert or snow or jungle planets.

Wrapping around a globe: These would be perfect for wrapping around an icosahedron, but the program could be tweaked to generate spherical data and probably a suitable 2D projection for that too.

Layers: Probably not; it'll just be one run, one planet.