I suggest that the formula for beehives and their wax be modified some to allow for higher qualities of wax to be possible (not easily possible necessarily, but easier then it is now). This, therefore, would allow players to spiral leather like we are able to spiral metal.
This would allow for a nice sink for leather, just like it is for metal. Currently there is nothing to use leather for after a certain point, but with this you could continually make hardened leather which you use to make more hardened leather to spiral it higher and higher.
Only disadvantage I could see with this is that it might be too easy to do, since it wouldn't really have the randomness that metal spiraling does.