Bigger Bonesack: This version of the Bonesack can hold up to 28 more bones, like an extra inventory.
Blighted Bonesack: This version of the Bonesack can hold an unlimited number of 1 type of bone, but the bones can only be inserted or extracted either at a Bank, or in the Wilderness.
Additionally, if you are using the Wildy Altar, the Wildy Altar will automatically start using bones from the Blighted Bonesack if there are no bones in your normal inventory. It will also use them 1 tick faster.
It also has a "Quick-Empty" option that fills your empty inventory spaces with bones, but can only be done in the Wilderness.
This would free-up all of the inventory space for an anti-PKer to be able to gear up, while they train prayer in the meantime.
How to obtain Bigger Bonesack: You must kill the "Skeletal Horror" boss. This is just a revival from Pre-EoC RS2 of the old boss, shown in the image below.
How to obtain Blighted Bonesack: You can get some kind of bonesack-blighting item from a new skeletal monster in the "Bone Yard" part of the Wilderness. You will then apply it to the Bigger Bonesack.
The point of this item is an examination of the actual gameplay of the Wildy Altar.
The Wildy Altar encourages doing runs using the Burning Amulet with only 1 inventory of bones, and then dying to restock and repeat. Doing this almost completely negates all of the risk, making the reward completely free. I would know, because I did it to 95 prayer.
It actively discourages tanking setups because the 1 inventory method is faster and less risky and bones in your inventory take up the space of food anyway.
It tries to encourage holding a lot of noted bones and unnoting them with the Zamorak mage, but you get interrupted by PKers just enough that you would never ever want to do this unless you're on a super low level account that can't be attacked. It's just overall worse to do this over the 1 inventory method.
It actively discourages Anti-PKing while actually utilizing the Wildy Altar, because if you fill your inventory with bones, it takes up the space of carrying food and equipment.
Some sort of bonesack was a common suggestion in the official OSRS PvP feedback Discord, and I think it would be great for enabling the styles of play other than the 1 inventory method.