Not in any particular order; just as they come to me (some of this is hardware, some software):
3. The upgrade process needs vast improvement. I'm not sure how, but it needs to be improved.
4. Workouts should be more flexible/give more options (such as support for haptic feedback of running intervals - run 3:00/walk 1:00; sprint 400 meters/easy jog 100 meters; etc.). Workouts should sync with third party options such as Strava without a third party app. Workouts should be visible on the watch itself when they are complete. And there needs to be an option to turn off the dopey 3...2...1... countdown without tapping the watch. You should be able to do almost everything with the button(s) and the crown without having to tap or force-press the screen.
6. The next Apple Watch should figure out an optional (not default) way how to show the time always, not just with a tap or wrist raise. (I'm fine with existing models not supporting that feature.)
9. For the benefit of outdoor workouts watch-only, the watch should show when there is a definitive GPS lock so you can wait to start the workout until location is connected.
11. I don't like most of the watch faces, and would like to see more high-quality versions without the Disney tie-in.
12. I want the control center to be customizable - allow me to remove icons I don't want, and re-position the ones I use most often, or learn the ones that I use and move them toward the top.
13. I don't like the off-center Digital Crown, and that there is only one hardware button. I think that a second hardware button should be added, putting the crown in the middle, and allowing the two buttons to be used in the workout app for start/stop/lap etc., and allow for a hardware button press in other apps don't access the dock or bring you back to the watch face.
14. The sales process: watches should be a la carte, so you can buy without a band if you already have some, and allow you to buy a watch with the band of your choice.
I agree with @
tadpoles about scratch resistant crystals. The should all be sapphire.