Couldn't find this via search, and I could see this being a total pain to implement, but figured I'd throw the idea out there.
It'd be nice if you were allowed to unequip a bindle/sack if you have enough spare space, and the items would automatically rearrange.
i.e. this:
when unequipping a traveller's sack (in this case), automatically becomes this:
And of course if there isn't any room then you'd just get the error you currently do.