>But unless there's a usecase for GUI elements being hit, excluding them on the basis of them being too far away on the z axis makes sense.

And what about that GUI element that I mentioned earlier? The marker around the vehicle that has to be behind Clonks but in front of vehicles?
I am pretty sure something like that would neither be rare not the only case where you need different behavior.
I don't think just adding more categories would solve the issues we have with categories at the moment.
I stand by my point

PS: I mean the whole flaw in the category system at the moment is that the order of objects is always linked to ingame behavior (You want an objects behind Clonks but not static back? Too bad, it's me. Categories.). And I think _that_ is what we should get rid of. Not just introducing more "categories" that actually do the same (but you notice it later since you have more now)