The ability to pick what magazine a weapon has in the Virtual Arsenal (or to be more precise, Eden Virtual Arsenal)
As it stands currently, if I want a unit to spawn with a tracer magazine without scripting, I have to close the editor, open up the virtual arsenal, equip the tracer magazine, try the loadout, unload the present magazine, load the tracer magazine, close back to the arsenal, save the loadout, close the arsenal, open the eden editor, edit the loadout of the unit and then load in the loadout I saved in the Virtual Arsenal, for the unit.
One neat little way of indicating what magazine is currently in your weapon without having to make a whole new box could be putting a star infront of the name of the magazine. (Putting the star afterwards might make the star disappear if the magazine name is too long)
Perhaps you could also select the magazine by holding ctrl while pressing the plus, the way you'd normally add a magazine to an inventory.
The latest devbranch update touched a bit on the virtual arsenal with some good changes.