The ability to declare one item as interchangeable with another in crafting recipes.
For example charcoal => coal. Jungle Wood => Oak Wood.
Also these would be one directional. So the above declaration does not mean coal => charcoal.
Exact matched would trigger first (so a recipe could require charcoal specifically in the above example).
Justification and Use Case:
As exampled above Minecraft already has analogues built in. Charcoal => coal. Decorative sandstone as regular sandstone. Various wood types etc.
As more types of wood are created and more recipes involving wood are added the ability to declare your item as substitutable for another means it can be used sensibly in recipes you don't even know exist.
As with much of the API this would mean a rewrite of the recipe handling code..