without being concerned about changes that would need to be made to the present gui, i'd suggest getting rid of the grid in inventory screen, at least in most cases, because in most cases it imho does not make sense.
there is already a no grid like thing at the hand slots.
now, without being concerned about the changes and the principal difference it would make, or the lack of it, i think it would look nicer if the weapons would just be attached to a point, rather than into a box.
where this came from is that, say, you might have a rocket launcher on your back.
assuming you have some kind of backpack, i wouldn't put the rocket launcher in the backpack, but it would rather be hanging on my back.
i believe this connects to the current discussion in the sense that 'hanging on your back' is a quick access slot, while the backpack, if there is one, is not.
'hanging on back' would be a quick slot where you can put one item no matter how big, but also no matter how small.
provided the item has the appropriate belt (or whatever you call it), meant for carrying it on the back, like most bigger guns do.
the difference is that you can put no more than one item there.
also, when you put something on your belt, like a secondary weapon, it usually comes with a holster or sheath appropriate for attaching it to the belt.
so, if your secondary weapon is a monomolecular knife, it will not be in your backpack, but in the quick access slot on your belt instead.
as there is no grid, you cannot put a pistol there _and_ fill the rest of the space with grenades.
the grid might be appropriate for the backpack, as backpacks might actually have something like volume.
accessing the content of your backpack would certainly take more time than accessing something from a quick slot.
and, probably, with the use of the quick slots, the inventory screen could even be reduced to just the backpack box, if simplification is needed (bit sarcastic here).
there would be a requirement that every weapon either has its appropriate holster on belt, or else can be carried on back.
this requirement would remove the artifact of dropping things on the ground or putting a knife in the same slot where there was previously a pistol.
hopefully this kind of more strict system will help to simplify equipping and removing weapons if the shortcut system starts to appeal to someone.
i mean, if the pistol in hand is being replaced by a grenade, there is no question about where the pistol would go -- it would go to its dedicated holster.