It’s hard to say what unique conceptual space Heward’s Handy Haversack occupies: it’s like a bag of holding but… on your back? Its main schtick seems to be that it’s more convenient. The level-up powers I assigned to the Haversack mostly have to do with convenience, although the bag and pack powers are pretty much interchangeable.
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.
Jedd the Hunter’s Handy Haversack You can put one medium or small person inside Jedd’s haversack: they must be dead, helpless or willing. If they are Dead, the sack acts like a ritual of restful repose. if living, they will be put in suspended animation until they are removed from the haversack. A suspended creature will occasionally become conscious, and if not otherwise restrained, can make a Hard escape check once per day.
A willing subject may ride around in the backpack with just his head sticking out, giving helpful advice like “Look behind you!”
Handy Haversack Familiar
This sentient haversack is loyal to its master. It may deny others access (opening it against its will requires a DC 22 strength check), or open to reveal a normal, empty bag interior. Once per turn, it may, unaided, either drop one of its items, or pick up an item in its square. It may also move, with a movement speed of 4, although it will generally not do this if observed by anyone but its owner.
Handy Haversack of Useful Items
Once per day, the Handy Haversack can produce an item that was never put into it. It can produce any item from the PHB1 equipment list, that is small enough to be removed and that has a price of less than 50 gp. The produced item will disappear at the end of the next extended rest.