I don't know if this have been suggested before or not, but I really think that the grand looting bag should get a filter, so you can avoid the instantly filled bag. This is something I know everyone or atleast most of the people that have the bag already want (Not that it's a lot of us).
Before people start saying that'll be way too overpowered. Keep in mind that the bag is estimated around 5-8b worth or even more. This all depend on how nice deals you get for the items you put in the rune coin exchanger. So the bag is the price of pretty much BIS gear.
Example: You go kill a few dark beasts with your Grand looting bag, but you are unlucky and get 6 caskets from one, and because you're a supreme or prime mode, you get double drop. That's instantly 12 slots in the grand looting bag filled up.