Many specialty bags have been suggested, like one for seeds, one for food, one for drinks and so on. Right now we have the Spice Rack, the Alchemistry Kit, the Elemental Pouch, a couple of Toolboxes...
Some of them could fit into a bag slot, not only a base slot. Like second-tier bags.
The Spice Rack, the Alchemistry Kit and a seed bag, for example, could believably do that. I guess a toolbox or a bag full of food wouldn't.
This would be a nice way to have more specialty bags without losing inventory space. In fact, increasing it.
What do you think?
ETA: A simple way to distinguish them can be this rule: If it's not a "bag" (as in "it's a Rack", "it's a Kit" and so on) it fits in a bag. Then again, what to do with a "box"?