I think it would be, if at all possible, incredibly useful to be able to create custom knappable items.
To put it simply, I would choose what item can be knapped, wool for example, how many get consumed when knapping (just like how clay consumes 5 but rocks consume 1), whether or not you can try again (like clay where if you fail you don't lose all 5 clay), if you require a tool (like leather with knives), and furthermore what their outputs/recipes are.
Additionally the ability to remove existing items like rocks/clay/leather and add to those categories, Eg: adding PrimalCore rocks to be useable as igneous/metamorphic/sedimentary rocks.

This just improves compatibility and customisation, nothing more.

There's two things here:

• making an existing item usable as an ingredient for existing knapping is doable - ideally this will be accomplished with an ore dictionary value (if it isn't already) (Edit: added to #442)
• making a new knapping gui / system altogether can be done via an addon, using the API to add a new knapping type and implementation

Vanilla clay doesn't have an ore dict entry by default. The only one that needs to be updated to ore dict is charcoal in IPlaceableItem.

We can add clay to vanilla clay, fireClay to ours. That should solve both situations