This could technically be a "backpack" equivalent to a second inventory, BUT would only hold pixelmon items; not anything from vanilla minecraft. Also, said pixelmon items would default to this bag but still go into the regular inventory if the bag got full.
This would let people carry all their pokeballs and use them in battle, all kinds of potions and healing items, and even an assortment of held items; all without cluttering up their main inventory. Having to choose between this and that has been one of the main issues I've had to deal with when playing pixelmon. Why even have all those nifty healing items in the game if I'm never going to use them because I don't have space to carry them and do whatever else I'm planning on doing?