First of all, I think this phone will be released next year, in 2015. It has perviously been speculated that Apple dislikes it's 'S' branding as people do not see it as an entirely new handset. However, releasing an iPhone 7 next year would fix this.
I think Apple will stay with a fairly similar design, keeping aluminium: Although, yes, I understand people have been complaining about this due to the recent 'BendGate' issues, what people don't understand is that it is not the metal causing the phone to bend. It is a particularly week spot where a titanium reinforcement ends. Apple loves making Aluminium mobile devices, and I cant see that changing.
The screen size will likely be the same as the 6 and 6 Plus. 4.7 and 5.5 inches. That is unless Apple reduces the bezels.
Apple could pull off something revolutionary, however. Removing thee bottom and somehow implementing Touch ID in the display. Whilst doing this I could see them also reducing the top bezel and increasing the screen sizes to something like: 5.0 and 4.7".
Other than that, I think the main specs of the phone will be a followed:
- Vastly improved camera. With optics fairly similar to the current generation, the major change would be it's resolution. Potentially, I could see Apple upping the resolution to 16-20MP. Other features could include the ability to record at 360FPS slow-mo and 4K video.
- An improved FaceTIme camrea with ability to record at 1080p and take higher reolution photos (3-4MP).
- A new, faster A9/10. With a 14nm design, the chip would be more power-efficent and potentially have a much higher clock speed. I would imagine, however, Apple sticks with it's dual-core design.
- 2/3GB of RAM. It is already rumored that Apple's next generation iPad Air may feature 2GB of RAM. I would expect Apple to follow it's move with the iPhone 6S/7.
- Inductive charging. The implementation of this in the Apple Watch is a major indication that this feature will come to the next generation iPhones.
- iOS 9/10. Potentially some major design changes and futher expandability.
As we are already seeing new iPhone rumors (the A9 chip). I think this next generation iPhone will be something amazing and, due to the latest bad press, will have to be.
If Apple can 'wow' the public, much like they did in 2007, once more, the iPhone 7 will be a major success.