The computer hasn't really changed much in the last 20 years other than size weight and speed (along with cost of course) so I don't think we will see much innovation in phone design.
The fact is there is a lack of innovation at the moment, possibly due to this nonsense over patents which companies like Apple seem to be buying up or claiming they own.
Look around at the phone market and it's a 3-4 inch glass touch screen phone running 2 or 3 OS systems.
WE do see phones trying to be more like small tablets (the Note) and tablets getting smaller, but where we go next is hard to say. I suspect the design of the typical smartphone won't change much, but the way we use phones might.
Already we see phones able to predict what you might be wanting to do (for example if you're near the supermarket) in the future, monitor your health (anyone remember Stargate with the Tolens who all had a remote health monitor) or work out if you're feeling stressed, more integration with other devices like the car (my car automatically connects to my phone via BT, so the next obvious thing would be to automatically tell my phone when I refuel, what my MPG is and contacting the garage to book the service etc)
So I think it's more likely to be that way than actual design.