Id like the ability to charge travelers to use the waystones using an ingame item defined by the waystone's owner.

I propose two modes:

One mode is direct travel tax, where you lose the item or items similar to the current XP based travel mode.

Another is a Key based mode where you keep the item you use to unlock the waystone.

Ideally adding an intergration directly with the numismatic mod for the the direct removal of currency would be good.

Item costs are already possible in recent versions using the add_item_cost warp requirement. The next version will also add a has_item warp condition that can be used for key-based use cases.

For Numismatic, I believe I've seen an addon in the past that adds support between the two mods.