TL;DR
Allow the player to control from and to where exactly does the inserter put the item.What?
Add options in the inserter GUI to select whether it should be putting hand contents into ingredients, storage, module slots, fuel slots etc. and picking out products or other things.I suppose that it would be disabled when the inserter is not directed towards a valid entity and that it would be a checkbox list, but those two are implementation details that aren't that meaningfull.
Why?
This could allow automating more contraptions, such as a smeltry that swaps the modules automatically from quality to other when too much of given ingredient in quality was manufactured, help in controlling modules of auto-malls, maybe automatic tank refilling station or a belt that fills slots in tanks equipment grids.Additionaly there already is an option to move modules from and to beacons and I find it unintuitive that this is the case for only one entity type.
I've suggested the same in this thread and it seemed to get kind reception, so maybe @SupplyDepoo, @myridium and @mrvn can provide further reasons and examples - specifically mrvn mentioned that "It would solve several other feature requests".
Finally I'd like to clarify that I do NOT mean anything like Bob's Adjustable Inserters, as this would be detrimental to Factorios gameplay in my personal opinion.