as far as i know, this is completely out of the question with the current game engine. i believe it would take a complete rewrite and whole new engine to implement this.
That being said, i don't think this is a bad idea, but just one that isn't going to happen until i get 600 supermodels all wanting to lay me at the same time.
Not really they can mix up with a voxel based terrain. It would SUCK hard but its doable :P.
For example, they could generate a voxel based terrain and let only that terrain be "diggable" while the rest is pre generated with the current terrain generator and its fixed on the map. Argh i'm terrible in explaining things sorry.
I dont like the idea of diging anymore. After watching how they are implementing caves i think its the best way to go. a dungeon,tunels and caves generated with the map is better.
A game like rust the exploring part is much better than digging.
PS: I suck at drawing too.
Edited : Somenthing like this