Bag of Holding Idea Help.

I'm starting a new campaign today and I'm the DM. My players are starting at level 1 and having jokingly asking for a bag of holding constantly. So I had an idea to give them a "cursed" bag of holding. The curse being anything that they put inside the bag of holding will disappear, but what they won't know is if they put enough items in the bag eventually it will throw out something useful for them. My question is should I have something ethereal live in the bag or should it just be an offering bag of sorts?