I would love if you could edit a marquee in 3D, like revit's section boxes.
Sometimes you want to show diagrams or drawings of the building cut on different sides, and this would be a useful tool for that.
Images below show the section box tool in Revit.
I use both Revit and Archicad daily, and while there are lots of stuff I miss from Archicad when using Revit, this might be the number one tool I'm missing from Revit in Archicad.