My idea for an improvement would be introducing a rectangle/polygon selection that will only add and one that will only subtract elements from current selection.
Currently, we select many elements by using rectangle/polygon select and holding down SHIFT key. This way we can both add and subtract elements from current selection.
The problem is that, in fact, holding down SHIFT only reverses current selection, so anything selected gets deselected and anything unselected gets selected. This doesn't give us full control and possibility to quickly add or remove many elements from the selection set. Sometimes it gets really annoying when we want to expand our selection but accidentally mark some already-selected elements, and they get deselected.
Many other CAD programs solve this by adding items to selection by holding down SHIFT and subtracting from selection by holding down CTRL (or opposite, or CTRL+SHIFT). I would gladly see a similar behaviour implemented in ArchiCAD.