I came to the comments to post exactly that. This mod would be great with further tesselation of the terrain, or rather smoothing using sloped and curved block types to create a more natural terrain. The game engine isn't built with a subsystem for fast partial removal of nodes though, so digging into a world still made out of cubes would be unsettling. There is a comfortable simplicity in cubic voxel worlds.
You may activate the smooth rendering from naturalslopeslib settings to make slopes look like actual slopes instead of stairs. This is not enabled by default because it breaks the blockish look that most game uses and it comes with some issues about height estimations and passability. But it's still there.
Even if it could look great on ideal cases, I won't be including more shapes. Cutting only the upper part in 4 seemed to me a good compromise for ease-of-use, performance and consistency.
Edge detection works by checking only about direct neighboring node presence or absence. It would be painfull to check for the further neighboring node and handle a lot of more cases. And if not all the neighboring nodes are sloped right, it will most likely add roughness than smoothness. Also the division of node already includes some issues about not being able to put things on the half-height (like putting things over or under slabs), smoothing slopes even further will double the number of cases where this issue arises.
But I understand why you are suggesting it. I agree a really smooth terrain would be very eye-candy.