Backpacks that provide some extra space in your inventory, and are equipped in your gear slots. but either take up more than 1 slot in your gear section and or only usable without a chest plate. to try and keep it used more for farming and less about wearing while roaming.
It would also be good if it made movement speed slightly slower, however i feel like making them as slow as wearing heavy armour would lead for it to be never used as it just wouldnt be enjoyable or any more efficent unless you used a horse or any vehicle which would then just enable zergs to effect how they are used more.
I have seen other people suggest that there should be “levels” to the backpacks, where they offer more space but at a higher cost and at a slower rate of movement, which i think is a good idea as long as the movement speed isnt reduced too heavily. In addition to the slower movement speed, i also think it would be a good idea to have them slightly lower your jumping height, to try and prevent people bringing them to raids, as i think that if they are introduced to the game then it is best to try have them as a tool for farming. As i think that the backpack may be taking away from the current balance of the game if they could be used to assist in PVP, moving loot etc.
Similarly, i think that they should be set to only carry certain items that you find when farming. Food, comps and uncooked resources, primarily because of my opinion on it being centred around being a farming tool. However, this might not have a massive impact on how people use them as they could carry other items in their regular inventory slots and move things around.
Let me know in the comments any different or additional ways you think the backpack could be balanced to prevent it from being too OP :)