Suggestion list

1. The "Input Barrel". This barrel would take hopper / pipe input and distribute it to connected barrels.
2. Some "Dummy Barrel" to connect barrels which are not connected visibly or around corners. Only needed when the first point becomes reality.
3. The "Throwaway Option". Add a lava bucket to a barrel (in an anvil) and it will destroy exceeding items. This one is my favourite.
4. The "Output Barrel". Only needed when the first point comes into live. This would be for automation only and be an interface for other storage / automation systems and simply act as a very big barrel which offers everything what is available in all connected barrels to the client.

Heavy automation will come in near future to 1.13. Mass storage solutions will be needed. That way Stockpile would keep up with still late competitors. But they already have huge 1.12 mods for storage. Stockpile could be an early game storage solution which would be able to outlast end game stage.