Object grouping feature would be very useful. Not to confuse with object folders, not the same thing.
If you group 2 objects, the first select click on any of the objects on the 3D view would select both objects, the second click would select the next group or the next object inside the first group. No limit of hierarchy deepness.
This would be very useful for composition, natural selections. When I create complex parts, sometimes I want to click a set of multiple objects directly on the 3D view instead of trying to look for the folder that have those objects. Sometimes I have dozens of folders with other folders inside it and trying to find the folder of the objects that I want to select can take a LOT of time