With composites the weight disadvantage will be reduced and a twin aisle makes more sense to move the C of G a bit closer together.
I think many composite parts such as the fuselage don't scale down that well. The skin thickness of the 787 will probably be the same as on a narrowbody. It made MHI move away from composites for their MRJ. (They make large parts of the 787)
High mounted rear engines will mean the cabin is lower to the ground and it will have quicker boarding (twin aisle will also help) and lighter stairs.
I've seen the designs. From what I heard authorities object. They find the risk of engine #1 taking out engine #2 too high. They want something inbetween..
Range and seating capacity should be less than the a320, but forget 700nm, thats for rail travel. 150 seats and 1500 nm range.
I notice many people tend to pooh-pooh anything with a max range below 1500NM.
In reality, it's where 90% of the action is..
