I was thinking that it would be an awesome idea to have framed backpack items added to the game, along with other potential 'bags' that mitigate the weight of the items we carry. These specific bags would allow characters to carry more weight, or move faster with the same amount of weight. This mitigating effect would only occur with items that are inside the backpack.
A potential add on to this idea is 'overencumbering' the backpack itself. After a certain weight, the backpack will loose most of its mitigating properties-so while you're still able to carry more weight then in a normal pack/bag, the amount of mitigation would go down.
EX: x= backpack weight mitigation, w=item weight, Bw= bag weight (the weight added to character to effect encumbrance)