Everyone is into terrain editig right now it seems.
I'm also working on some terrain based editing tool. But it is mainly for level creation for a specific game and thus only allows basic terrain modification like influencing shader settings and placing objects.
Other stuff like setting up a timeline and events is special game content.

The terrain itself is created via heightmap which cannot be modified in the editor. It's possible to pick different skins, though.

Currently not much is working as I was struggling with the Windows.Forms (.NET) integration up to now. But it seems like I got it stable now and can start adding functionality.