I think for series 4 we might see one or more of the following:
2-3 day battery life
Thinner profile
Always-on display option
FaceTime camera (probably something relatively low-res)
Faster - supporting better graphics, animation and Siri functionality
Improved iPhone-independent app functionality (LTE-Kit API or something)
points 1 and 2 above will occur with a larger size: 60-80mm like those in fashion boss watches.
This will allow for a larger battery (higher capacity), more RAM and chips, etc.
point 3 - I still hope this is optional. Maybe greyscale for time/date/weather/1 or 2 complications that provide updated data every 2-30 mins (stocks comes to mind). Also most likely limited to specific watch faces at first. that is unless 5 day average battery life will occur across teh board.
point 4 - camera on your wrist? Not even Samsung was crazy enough to follow through beyond a protype back in 97 (they may have again but that didn't go so well back in 2015/2016). What kind of quality should we expect VGA resolution like the first camera phone that debuted with an add-on module (Ericsson T68m/Sony-Ericsson T68i)?? No.
Point 5 ... what exactly are you referring to for the watch to be independent on?
- mini app store?
- iMessage iCloud integration for real-time reflection of messages (iMessage, SMS, Text, MMS)?
- Apple Music local storage of songs favorited?
- W1 chip licensed to medical accessories (hospital medical appliances, weight/BMI scales, diabetic perosnal equipment, etc)? Oh yes!
My addition ... a heavily ruggardized version for the extreme sports or military officer.
PS: By version 8 we may see iOS directly on the Apple Watch by then.