I guess polycarbonate would be a bad choice for the body as its extremely lightweight. Because if the weight is less then there is more chance that will fall from out hand. But it is true if they make it larger device and subtract weight then yes that will be well enough and will be a great choice. I would not consider to have a larger screen as that has its lot more disadvantages. But if you think about the overall design and concept then its really simply mind blowing but i am not sold on Apple making a larger screen that would inhibit one-handed use. You can visit ADR studio's site for more images and view more great designs.
Source: ADR Studio