I think that it would be convenient to have EquipmentSlot versions of Inventory#clear(int), Inventory#getItem(int), Inventory#setItem(int), etc. Alternatively (or additionally), being able to get the slot index from an EquipmentSlot would be useful, though the use of EquipmentSlots in armor stands might be an issue (don't know enough about armor stands to comment on this).

Another addition that would be cool to have would be something like setSlot(EquipmentSlot, ItemStack) in PlayerInventory, EntityEquipment, ArmorStand, etc.