I think backpacks are mostly fine now but I wouldn’t object additions. All of those backpacks already exist (except hiking), and I’m not sure if their stats differ much from the current ones.
Your MOLLE suggestion could be interesting. The MOLLE variations could also come as crafting recipes such as:
MOLLE version A
Required items: MOLLE Backpack + MOLLE pouch
MOLLE version A would give increased carry capacity over standard MOLLE backpack, with minimal increase in encumbrance.
MOLLE version B
Required items: MOLLE Backpack + 2x MOLLE pouch
Even more carry cap and increased encumbrance. And so on. How many extra pouches should a MOLLE accept?
Duggel bags are bit problematic. The current duffel bag provides 30L. Would going above it break balance? Even with encumbrance? 150L would certainly seem like too much, but on the other hand, then weight would become a limiting factor. Could be interesting. The encumbrance distribution really depends on how the bag is supposedly carried - on the shoulder or in either hand. I think in either case, it should completely prevent the use of the said hand.
I’m not sure about hiking backpacks if we already have MOLLEs. Wouldn’t object it either, though. Hiking bp is civilian, MOLLE is military gear, and they should be added to loot locations accordingly. MOLLE should be more durable.