Would be awesome if we could access all the inventories of our retainers from one interface, instead of having to call them back and forth individually. Even in-game that would make sense: Our retainers manage our storage, but which one we call to fetch us something does not matter.
The same way, selling something from the retainer inventories with a specific retainer would happen through one interface as well (instead right now sometimes calling retainer #1 to fetch the thing and then calling retainer #2 to sell it).