Wondrous Item, Rare, Requires Attunement (because commitment means something)
This bag is exactly like a bag of holding, except the bag has gained a small measure of sentience. Anytime you try to interact with the bag, it will have something to say about it. Like a jilted lover, it has issues trusting you now. You must succeed on a DC 15 Persuasion check to convince the bag to trust you.
A creature within the bag that can hear (though it need not understand language), must succeed on a DC 12 Wisdom saving throw at the end of each minute it spends inside the bag or become more and more sorry for the bag and take its side over yours.
If you ever unattune to the bag, and later reattune to it, all DCs go up. Because how does it know you won't just leave it again?