TL;DR
Chest-like entities that attach to the side of your factories to expand internal storage of the factory.What ?
I would like there to be multiple tiers of Factory Reservoirs that you can attach directly to any empty directly-adjacent side of your factories. These would expand the internal storage limit by an amount later to be determined. (Either multiples of the base storage amount or a set number per tier) Rather than pull items from this reservoir, inserters would continue to function normally; pulling items directly from the factory. Inserters would NOT be able to pull items from reservoirs directly, they would need to pull items from the factory. As many reservoirs could be attached as there is empty sides to attach to. They can NOT attach to each-other, they must be in direct contact with the factory to work. (Up to a max of 12, as that is the limit of potential attachment areas.) If multiple factories are within range of the same reservoir, you choose which factory it attaches to prior to placement via snap rotation. If a reservoir is removed and the items stored within the factory exceed the new storage limit, the items will be transferred into your inventory, else they fall to the ground.These reservoirs would NOT be able to interact with a logistics network. They are essentially just a larger storage space within the factory itself.