Before I get too far into the must-haves, I'll let you know my dream: it's something like a cuff, where the whole thing is a screen wrapped around my wrist. Maximum possible screen space. I don't even mind if it's two or three inches wide.
But these are the must haves, meaning without all of these requirements I won't even consider it:
Waterproof
Notifications from all apps (customizable)
Multi-day battery life
Steps
Also really nice to have, and would be heavily weighted in a decision:
Heart rate
Apps
And back to dreaming about the cuff:
Utterly unbreakable
Phone-free independent operation
Tether other devices (ex: tablet or laptop) to cuff, so cuff is communication hub.
The cuff replaces the phone. On wrist it sleeps with watch/jewelery face. If you take it off, it uncurves into a flat screen.
