Yeah, I think the jump to Series 4 will be an interesting one. It will give Apple to opportunity to focus on what they WANT to rather than what they NEED to. The low hanging fruit (GPS, LTE, and much quicker processor) are out of the way.
Some of the possibilities include:
1) Redesigned watch: Not really sure what this would entail because I don't see it going round, but Apple has had several years to think of how to enhance the design. I don't really imagine Apple's ready to make it thinner yet.
2) Low power processor: I know the processor is already pretty low power, but series 3 seems to be quick enough for the moment, so I think the primary focus of the next processor will be on reducing power consumption. Which would allow...
3) Always on screen: This will likely be some combination of optional, select watch faces, and/or select complications. I would love to be able to set (say) time and weather as "always on" complications and have the others appear on wrist raise
4) Improved sensors: HR sensor needs an update, in particular for always recording HR (A la fitbit). Possibly GPS/LTE can be improved/reduce power consumption. Perhaps more accurate movement sensors to help with sleep monitoring/stand/activity
5) Go larger: There's no way apple is going to 60mm-80mm I saw discussed, but I could definitely see Apple add a 48-50mm option. Or perhaps instead of adding a 3rd size switch from 38/42mm to 40mm/46mm, especially if Apple is able to reduce bezel size
6) Add a camera: Oh please God no
7) Updated wireless standards: possible improvements to wifi/bluetooth protocols
8) True wireless charging: if you could be ~8ft away from a charger (look at WiTricity), then it could be charging while tracking your sleep in bed
Sounds like a pretty good update to me
