As long as our magical and paranormal powered backpack can carry a volume of material allowing us to carry and build the Eiffel tower, we will be forced to create an item with magical and paranormal powers, called the 'Claim'
Then, this claim will create ridiculous situations, with players structures impossible to assault, to destroy, and completely unbalanced PVP.
To rebalance all that :
1- Start by adding a weight limit and a volume limit to the backpack carrying capacity. (volume of 1x1x1 Max)
2- Add a sunk mechanism if the backpack player is heavy.
3- Now, a player can't swim across a douve with building supply.
4- To fill a douve, a player must dig a cube of 1x1x1, and carry it to the douve, 1 by 1.
5- Add a falling mechanism to unfixed structure, turn structure impossible to fix on concrete, then people cant empile frame to climb.
6- Now, people have to fill the douve block after block, then they have to build a siege ramp block after block.
7- You can remove the claim, he's now useless.