Writing up new magic items is fun; using them is a completely different story. This one came out of a whimsical conversation with my significant others:
For referees: the Bag may be played as a straight-up magic item of uncertain character, but it is suggested that you allow the bag to demonstrate signs of "indigestion" - occasional loud burps, the exterior of the Bag churning every now and again, minor rumbling sounds from within. For full effect, you can have the Bag emit a contented belch after disgorging its contents, followed by a slight sigh. Everything inside should be perfectly fine and unhurt. Do not let player characters use rope or other means of attempting to keep gear separated within the bag - imagine what it might feel like to the Bag!