i think the real iphone 6 (not the 5s) will have a 720p screen and obviously a higher ppi resolution (that means, maintaining the 4 inch aspect ratio). of course new CPU, GPU, 2g RAM, storage capacity starting at 32g, more siri features, NFC, 10 MP camera with bigger lenses (yet really thin) and the surprise factor.
For the body i think it will be a little smaller then the actual 5, with a thickness under the 7mm.
there will be a new battery tecnology (liquid metal) which will allow the iphone to double the actual battery life and still be really thin.