Wonder what retainers could do though in place of not increasing their inventory size. The only things that come to mind are perhaps increasing the amount of x item type they can hold, or increase the amount of items they can put up on bazaar. So if you gather a lot of lumber and find yourself holding onto a lot of it, when your retainer levels you can increase the quantity of "wood type" products held by 100%. Or increase bazaar size by 2.

Or maybe since we do have spiritbonding in the game now a lv could also open the option to allow your NPC to work on spiritbonding 1 piece of gear up to 5 pieces of unique equipment upon obtaining 5 lvs or something like that.

It would be neat to see what their original idea of it was though odds are they were going to be concepts to make finding items easier but seeing as that's alrdy done they pretty much have to think of new implementation for a retainers lv lol.