An accordian style compartment backpack, that converts to a rolling bag when fully loaded and heavy with books. This bag is aimed at high school or university students.. I think that the rolling bag conversion will be most popular with young ladies, though a material combination of solid colors could also potentially appeal to young men.
Inspiration: 
When I was a student, I would leave the house for the studio and be out and about for 12-16 hours at a time. I needed one bag to hold everything - like a "mothership"! - but by the end of the day it would be heavy with library books and supplies. 
The ability to convert the backpack to a rolling bag when needed helps save the arms and shoulders of tired students :o) The bag houses compartments for:-- laptop-- library books -- files & sketchpads-- pens & pencils-- lunch box (or make-up case!)-- wallet & keys.