I am sorry when this idea is already suggested, but I couldnt find it.
My friend visited me and I explained him Trackmania. When we were at the editor, he wanted to create a map.
After he finished his map he drove it and he crashed against a wall ( it was too deep) and he wanted to change the height. He asked me how he can mark/select a whole part of the map and I said, thats not possible.
So my suggestion
you can select a rectangular range by using your mouse while holding down the CTRL key ( for example )to draw a rectangle around the range. Then you can change the height or move it. I think thats a very important editor function.
I am interested about your opinion

Regards
Max