I've wondered why an airline doesn't try a "capsule hotel" style setup ever since I went to Japan.
http://en.wikipedia.org/wiki/Capsule_hotel
http://www.yesicanusechopsticks.com/capsule/
If you built a section of plane for it specifically, I bet you could fit as many people, and I'd love to be able to lay out and sleep. Maybe the engineering or safety wouldn't be possible, but if it was, I think it'd be fantastic.