IMO, just make every bag bigger by default. It's QoL, it doesn't have to be content/a grind/exclusive.
Huge Ditto There, HUGE!
It would also be cool if there was an NPC that could trade say 7 mini's for a rune. Give all those white crappy mini's a value and remove them from the market.
Another idea would be for Nicholas to trade miniatures for random junk (like Thackerey), with a chance of an untradeable Superior Rune of Holding. Possibly even in Pre-Searing. It would help remove minatures of the market, open up new interesting dynamics in selling/trading crappy mini's. People would still have to visit Nick with 5-10 mini's as the runes would be untradable. Possibly limit to 1 rune/week.
This is a fantastic idea as a stand alone.
Let's face it, the BD presents become useless after awhile.
With the age of the game, you finally have them all, and can't trade them because everyone else does too.
Comps have a mini dump file, and I have my own mini dump file, it's called the trash.
We need something else to do with these critters that would make the BD presents valuable in other ways again.
For decade old players, and those who have the ill luck of just getting repeat after repeat,
even an NPC just to buy the unwanted ones would be nice.
At least it's a couple of BD bucks to spend on booze or party or something else!