A phone with a roll-out screen along the same lines as that shows in the sci-fi series Earth: Final Conflict would be interesting to use.
This obsession with 'screens' is just a passing phase. I'm waiting for communication devices with wifi links to a visual cortex implant.
But seriously, the only 'phone improvement' I care about, would be for the service providers to offer a prepaid phone account in which they don't steal your credit balance after some small interval of time. Since I very rarely use the thing.
Terrahertz, the other fun thing with prepaid in Australia, if you buy a data block for 12 months, and a few months in they decide to change that plan option to 6 or 3! months, well you end up shafted, had this happen to me by 5 different providers multiple times,