I just weeded through a large swathe of my nine retainers that were completely full last week - takes time to go through everything and see what is worth keeping and what is not. Some random green gear piece from a retainer can be work over 100k sometimes, while most are sub 10k. Some monster drops are pretty pricey as well if they are rare or if someone has drained the market of them recently. Even when you par it down to things that are, to you, worth selling via retainer, takes time to price and take a lot more time for it to actually sell if it's not something of a mainstream item. Not hard to see how inventory can get way from you.

While I would love more room in retainers without necessarily increasing the number provided - I would be happier with some inventory management tools. If you search your inventory for an item and see 5 different retainers have a couple, and you have some, and there's one random one in your chocobo also - takes time to merge all of those together. Would love a way to auto combine stacks. Or a "Dump All" button to give everything in your inventory that will fit, top down, to your summoned retainer. That coupled with a "Sort and Combine" would be great - leave "Sort" as it is for those having specific uses for it, but with a "Sort and Combine" would help free up double or triple groups of the same item.

Tackle box for bait would be great too, like the key items tab.