Currently, custom containers can only consume the ingredients of its recipes and output the results of its recipes. It would be interesting to also allow recipes to upgrade the ingredients rather than replacing them.

With upgrades, I mean improving some of the stats of a custom item stack and incrementing some counters in its NBT to keep track of which upgrades it has had so far. The user would be able to add upgrade slots to custom container recipes. The user will be able to select which items are allowed in that slot, and under which conditions (preferable use some kind of NBT variables as input for the condition). And of course, what changes should be made when the recipe is executed (turn the item into a different item, upgrade attribute modifiers, change the values of the NBT variables...).

This task sounds very promising to implement, but also a lot of work.

This has been implemented in KCI 12