It would be great to have a slope levelizer.
The problem with slopes is that no matter what tool you use, you can only scratch the surface of the heightmap, which means that whatever change you do, your actually destroying how the terrain looks, for example: a river.
I am suggesting a tool which doesnt modifiy the surface, but modify the height underneath the surface.
My case: I´m creating a Arma2 map, Helmand Province. Helmand has a difference of almost 200m in altitude from its northeast corner to its southwest edge. Running down this slope, there´s a river called Helmand River. Arma2 does not support river, but it supports water at sea level. If my map could be levelized: to be flatened whitout loosing all its surface features, I could easily simulate the river at sealevel.