The title mostly sums it up.
Currently, we have a few things that are obtainable throughout ranking – Inventory Space, Storage Space, and House Commands. My suggestion is a rather simple idea, as for implementing – probably not so much…
Even though storage space is expanded, some players may choose to store items in their house, for example, as a shop. Another example of this is players using their house to put their item sets together, making it much easier to get specific ones when training (Such as a respective Pokemon’s family item). Perhaps, ranking up can allow you to drop more items in a single house – and say, over the course of getting to Guildmaster, a player can now store 60* items in a single house.
Currently, there are communal family item houses (LeoFam1-6, etc.). I strongly feel even though there’s a number of houses, that these sorts of shops and houses that are similar, could benefit from this – as well as those that like to use item for decoration.
*-Can be more, can be less. Just an example.