How would it work?
It would allow to use nodes controlling how terrain shape, textures and decorations are placed by layering and mixing various procedurally generated mask textures.
The inputs would be procedural textures such as perlin noises, voronois, fractals, etc, nodes would allow to mix them, blend, layer together to create interesting mask. The global heightmap would be usable input as well.
The nodes can affect following: terrain heightmap,
decorations - affecting how dense and in what areas are placed trees, grass and various statics,
textures - mask affecting what terrain texture is used
This would be pretty powerful in making world more interesting and as well user friendly.
Example:
