I would like the capability to drag an inventory tab to another tab—either in my own inventory or my retainer's—and have the contents of the two tabs swap. This would be very useful to keep lots of items in a particular category, such as culinary ingredients, grouped together, and be able to remove them from your inventory easily when you are finished synthing, then add them again later.

This may be a problem only for people who are pursuing most or all of the DoH/DoL classes, whose inventories tend to fill up, but I think it would be a useful feature for everyone.