I've been interested in small phone for a while, not because I want a small phone, but because I want a small battery with 4G, bluetooth and just enough screen to be used as a fall-back. We're almost moving past that form-factor.
At some point we're going to have separate things for input/output instead of just one black rectangle.
So when 'mobile' I use devices for:
- Audio (mostly podcasts).
- Messaging, email.
- Reading (news, tech stuff)
- Watching video
For those I need power, connectivity, visual output, audio output, and applications/processing power. But where each of those things happens is likely to change.
If you take the audio part, I have a bluetooth headset that I basically wear all the time, a SBH20 bluetoothed to my phone. It has a 6 hour battery (usage, all day standby), buttons for play/pause, FF, RW, volume, so the task of podcast listening is not done on my phone. I setup the subscriptions and order the list on my phone, but then it's all on a tiny device hooked to my collar.
In future I can see the visual output going that way as well, so instead of stuffing a 7" screen in your trouser pocket, you have glasses with an equivalent to a 50" screen in front of your face.
I've found that if I use a tablet for reading on the bus/train instead of my phone, I enjoy it more and my eyes don't go myopic afterwards (because I can hold it further away). If that moved to glasses, so much the better.
There are other things that need to come into play, but the first step is a 'small phone' and other devices for audio & visuals.