Certainly a system that takes into account the max portage capability - as currently anyone can fill up every space way above their limits (granted be slow as hell). Attempts at trying to add something heavier than their max carry capacity should result in an auto-drop.
Additionally, varied container sizes would be good. Originally BW had planned to have small, medium and large containers but dropped it - which resulted in the tardis effect (small boxes and large bags having the same capacity - ooh look, I can stick a polearm in a small box!).
Even without varying the sizes of the containers, a gross max weight allowance for containers would be even better as it would take care of varied container sizes without additional inventory sizes.