It would be an backpack that has the in-battle UI read it for items, and making them usable. This backpack could be 18 slots large, and store any item.
An alternative is allowing other modded backpacks to be able to have their stored items be usable in pokemon battles.
This feature is one that I'm sure isn't original, but could be made.
I am hopeful that this addition could be an great quality of life feature!