Goms,
Yes, good ideas. I've thought of some that. I have a stepped canyon type terrain which better demonstrates the direction this can go but I'm only scratching the surface. Probably today I'll post some WIPs of what I've been doing and you'll see that. The altitude thing is a bit tricky. I tried driving sizes by altitude and it had an unexpected result. It actually seemed to squeeze the shapes vertically - but maybe that's just due to my graph. This is in fact really cool but doesn't work when you move from origin if you have displaced the shapes. That's all a bit of a mess with dodgy graphs and I'll have to go back to study it.
The great thing about blending terrains by altitude is that you can apply quite extreme effects to one part of the terrain but this won't be all over the terrain. This is one of the key successes of using this technique. You can have very jagged type rocks higher up.
One thing I found was that the very regular unnatural shapes created by the voronoi is not a problem (I thought it would be) because when you add other functions to mess that up a little, it suddenly looks quite natural.
I saw your terrain on the terrain sharing thread and you did exactly the type of thing we usually need to do to get good terrain. It's an excellent terrain.