I would want it soft, but a little structured (like how backpacks have padding, etc), and definitely have pockets and loops on the inside and outside.
I would want a smaller, more flexible bag that fits comfortably inside it, containing only the essentials (if you're familiar with George Carlin's "Stuff" routine (you should be, it's funny), the small bag would be the "other side of Maui" bag) - pencils, sharpie, various tapes, etc.
That sounds like a really awesome project, I have to say! Have fun with it!
EDIT: All of these specs are second to, of course, the Mary Poppins/ Hermione-in-book-7 bag, but they haven't invented that yet.