I know the backpack should fit in the slot for the chest plate, but it would be great if we get an extra slot only for the backpack. For the example above the shield slot.

Maybe you can think about it and I hope it makes you not so much extra work.

Yes, backpack is cool but it has to compete with chestplate and elytra for equipment slot, which makes it much less viable.

This would go against the purpose of the item. We also don't take suggestions here. I recommend you post them on /r/quarkmod on reddit.