Hi Ulco, on a global scale that's all a bit tricky. Not technically, but artistically. Only a small fraction of the surface of a planet is covered with fields and only in certain climate zones etc...
If you use small scale shading like this and look at it from space, it just looks very unnatural.
I haven't even planned to have "fields", is just that very occasionally the voronois "hit" a slope constraint, so that the shape becomes obvious.
An interesting fact about the displacement: I actually wanted a few alpine shapes here and there but the alpine is so slow to render, and worse, it doesn't work far away from the origin. So I experiemented with masked and layered voronoi to create a similar shape like the alpine, just more coarse, and I use it to "influence" the build-up of the more roundish perlin mountains. You get a straight line every here and there, but overall I think it looks good. Alps look very voronoi-ish when seen from space - very similar. Anyway, I still have a long way to go to make the perfect procedural planet. I will probably never finish entirely. But at one point I am going to release what I have.
Cheers,
Frank