Doing that would require two things.
With really big worlds, you need a terrain system that recursively subdivides, because normal terrain LOD isn't big enough. With a normal terrain LOD system where the terrain is divided into patches, eventually you can zoom out so far that each patch is just one pixel big. So you need a different way of handling that.
You also need a way to warp the terrain to a sphere, and have it affect physics, raycasting, culling, as well as the rendering.
I plan on doing stuff like this in the future, but not with our standard terrain system.