The recent Winter Waltz event had a beaded handbag reward that looks like santa's red gift bag which, when held, was shown on the player's back. It looks cool. Though when a player is walking, the bag model moves in relation to the player's arm, so it's not stationary - though it looks normal when the player isn't moving. Then again, it's not a big problem, just minecraft limitations. Most of the beaded handbag models have been created for holiday events, such as Easter, Halloween, and Christmas which are themed for the respective holiday.
I agree that it would be cool to have an alternative beaded handbag design, but not to replace the current handbag, but to give players other options such as what you suggested with the adventurer's backpack. It would be interesting if they could pull off a fanny/hip pack model without it looking odd when a player moves, too. Imagine if they created other models like a sock, a duffle bag, or wheeled suitcase.
If they ever decided to create new models that players can buy with gold, then they could create a specific shop in Diagon that sells them instead of putting them in the joke shop npcs.
