I thought it would be cool if you sart out with a certain amount of item slots and different style slots (weopons, rings, etc..) then if you get a belt you get more slots and same as a backpack. They could even have different style belts that are made for classes, like wizards might have more slots made for scrolls and ritual components fighters for weopons. This is only to make the backpack and belt feel like they actually have a purpose. This would also be a much easier system than encumberance as you just write down the item where it goes and don't need to change numbers around everytime you do it.
Or you can do a Tetris style. Different items have different shapes.
The problem with that is if you fill up a row the item disappears.
I think the whole item system is balanced. you cant wear more than 1 item on you for one slot, simple and balanced.
And what are you talking about. there are a lot of useful Belts in 4th ED. look up waist items. and in the AV there is even more useful items.
and if you want magical backpacks, just get bags of holding. they don't take a slot and you put them on your belt or in your backpack...
or you can make a home ruling that bags of holding are the size of backpacks with many different pouches.
and whats wrong with encumbrance in 4th? you can carry up to 10 times your strength score in items without problem or up to 20 times your strength score with a hit to movement.