Let us link elements so that when one element is added, the linked elements are automatically added as well.
An example is: if we have a character that always wears the same costume, every time we tag that character with its "Cast Member" element, we also want it to be tagged with his costume element as well.

Final Draft has this feature.

Implementation-wise, my suggestion would be:
Below the "create element" and "edit element" panels, have a list — named "Linked" — where we can add/remove other elements to. There's plenty of empty space under those panels, so it should be perfect.