It's not unreasonable to think about 10, 20 or even 30 years with mostly incremental improvements and no ground-breaking changes in both smartphones and smartwatches. I list some examples below. Several of them are already under development, and some are already available but just not cheap yet.

- Batteries that can be fully charged in 5 minutes;

- Autonomy to go for a month without a charge, achieved by the combinations of new technologies in batteries, screens, chips and better software;

- Scratch and shock resistant screens that can be equally visible on daylight and at night, maybe by working like and colored e-reader screen when there's enough ambient light available, thus making reading more comfortable;

- Some kind of battery charging technology that improves over the awful usb connections that we have nowadays, whose cable connectors don't last much;

- Better cameras that rival today's DSLR. There are some experiences going on using several small combined cheap sensors in a single smartphone to achieve results in the DSLR range;

- Cheap and fast disk storage over say, 10 TB, as fast as RAM;

- Cheap and fast data connection at any location of the planet. Like streaming HD netflix in your tent in the middle of a blizzard in Antartida;

- High quality, small, wireless earphones whose batteries last for a week without recharging;

- Simpler, faster and more efficient development plataforms;

... And I'm sure you can add a few more. Each of these are changes that separately would not be considered exactly ground-breaking, but all of them together could bring mobile computing to a whole new level.