Predictions:
Thinner, sapphire on all models, new colours for Sport, new materials for Watch tier
Faster S2 processor
Same 1+ day battery
3D Touch (Force touch discontinued)
New health sensors (blood oxygenation and sugar measurements, EMG for measuring muscle activation*)
Dual lens camera hidden under curved glass. One pointing up for FaceTime, the other pointing forward for photos
WPS (instead of GPS) and open WiFi network. Will only work in select cities, maybe partnered with Starbucks
Open Lightning port for MFi certified 3rd party accessories
New branded bracelet and Watch face partners
A trade in program for Edition models
I think the health sensors are the killer app for the watch and for Apple specifically. They bought quite a few cutting edge sensor companies and hired quite a few experts -- too many to just put a heart rate monitor on the thing. There's a good chance we'll see sensor technology that nobody else has access to.