This image had me intrigued:
It looks like two semi-circles, the upper part for passengers and the lower part for cargo.
I was wondering, for short haul flights, could the lower part be converted like the upper part, so the plane could carry more passengers, like a basement stuff, no windows and lower fares. We could do away with cabin luggage and stuff to take care of maximum takeoff weight, and anyways my thought was towards short haul flights.