In AD&D, where your treasure haul is limited by encumbrance, you need a way to increase encumbrance limits along with treasure hauls. Therefore there are 4 types of bags of holding in the AD&D DMG, with capacities of 250, 500, 1000, and 1500 pounds. In 4e, there is no assumption that weight limits for treasure will be a regular part of gameplay. Therefore, bags of holding need some other, more 4e-friendly ways to level up.
While some items may get mechanically better (for instance, a +1 sword becomes a +2 sword), it's more challenging to improve items that don't have numeric bonuses. I thought I'd go through the Wondrous Items in the 4e Player's Handbook and give examples of how each could gain powers that reflect their history.
Pockets of Holding This bag of holding can be cut according to a specific eldritch pattern and sewn as pockets into six garments. Each pocket allows access to the same shared bag of holding.
The Pockets of Holding pattern was invented by an honorable adventuring party who wanted to share their healing potions, but later misused by a band of dwarven thieves who discovered their shoplifting applications.
Self-Holding Bag: The bag of holding-boosting ritual from Dragon 385 could as easily be a leveled magic item. This bag can be hidden in or removed from an extradimensional space as a minor action.
Seleris the Magician's Bag of Holding
If you put one hand in the bag of holding (a minor action) you can transfer items from your other hand to the bag and vice versa as a free action.
Seleris the Magician used to put one hand in his pocket while doing conjuring tricks with his other hand. Due to his Bag of Holding and other magical trinkets, he developed a great reputation as a wizard without ever learning a single spell.
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