Bag of homogenization, a magical bag that will convert the material of items placed into it to match the material of a stack of the same object type in the bag, if one exists, so that they can stack together. If this is unbalanced, or it’s desired to make it facilitate stacking even more, it can also remove enchantment and fooproofing from the items placed into it.

Deliberately dipping a bag into water wets all of its contents with no random chance of failing.

Grab bag: a magical bag that contains up to four small random items. You may take out any one item, and are allowed to view the contents first; after taking out one item the bag and its other contents vanish. Items cannot be put into a grab bag. If you tip it, one random item comes out and then it vanishes.

A magical bag which only holds gold and gems, but any gold and gems inside it don’t weigh anything.

Separate command to pick up an item and put it into a bag immediately after, in 1 turn. Or an option that changes pickup behavior to this effect.

You can put rocks into a sack, oilskin sack, or bag of holding and then wield it and swing it at enemies. (This uses flail skill.) Damage depends on the total weight of the rocks.

You can wear an empty bag over your head to blind yourself, but this sets charisma to 6. If you wear an oilskin sack on your head, it starts a strangulation timer. If you wear a bag of tricks on your head, you get YAFM and instadeath, or possibly it just constantly bites your head.