as I remember there are some hmp editing functions in WED too... but these default tools only toys, cannot provide realistic result alone. at the moment Terragen or L3DT or Earthsculptor is needed, but their export options usually don't work well with 3dgs, only with heightmap images that could result in quality loss...
so I think a 3DGS specific new tool is required, but that should generate realistic terrains, not only edit them, which is not simple at all (e.g. simulating erosion). and such a work should be promoted by Conitec somehow... (e.g. Unity has a 3rd party extension tool for this purpose)