With the expansion of inventory, but not retainer inventory, can become a future problem too. And we will have the same GearSets problems - you can't store all sets collection, so my suggestion has two aspects.

1. Add a grimorie Glamour (like many times we said), for completion it can be very very fun, people will run old content everyday to farm it, but in term of space, the gear can be registered and after discard (or consume by the grimorie).

(but.. if option 1 cannot be.. because of Retainer Bussines to SE) at least.. take this in consideration.. and with the new that no more space for Reatiners...

2. Add a Wardrobe with a max capacity of 40 - 50 gearsets, to each retainer, in this way we can have all space organized.... and maybe (only if SE) it could double the slot on retainers (letting it.. to not completed gear, and materials, and unique items, craft..)

And
Add FISHERS BAITS BOX (have all baits in one slot or something similar) xD

Thanks and sorry for my English ^^