good idea, here's some suggestions:
backpack straps that come out/tuck away (not just a shoulder strap, but backpack straps that leave your hands free)
but you should also be able to place food horizontally instead of vertically (vertically, the food melts on itself, yuk!)
plenty of room for larger entrees
extra pockets for silverware, spices, etc.
don't worry about so much insulation, but provide a place to put in ice packs