Most of what I want is software related: better notification control (allow tone choice, allow some with sound, some with haptics, some with none, some with both), better workout app (an option to get rid of the dumb countdown, support for interval training). I'd love to see some smart coaching for running/biking/walking/swimming/triathlon, either as part of an iPhone app or just on the watch, based on previous workouts and heart rate stats. Hardware buttons for the workout app as well, to start/stop or indicate a lap. Support to share workout app data to third party apps like Strava, Garmin, etc. The ability to pair with an iPad, if desired. Support for more/better watch faces (I'd be ok with aggressive vetting in the app store for these, to prevent people from making a duplicate Rolex face, say.) I'd love the ability to uninstall stock apps like iOS 10 added, such as the phone app. I will NEVER place or receive a call on my watch.
As for hardware, a sleeker design I think (though I'm fine with the current thickness), with a slight curve on the wrist side rather than being a flat slab of metal with a glass bubble with sensors in the middle. The sensor bubble is ok, just curve the case ever so slightly to make it more sleek. Perhaps a hardware button opposite the digital crown, or put the crown in the middle with buttons above and below (for chronograph support as described above.) Somehow add at least an option for always on time display, with an option to cut it when the battery is below a threshold. I'm ok with the rectangular shape. Honestly, for what the Apple Watch is, I prefer it to round.
I have no need for mobile data, but ok with an optional model with data if people really want that.
I'd only buy it if my Series 2 was lost, stolen, broken, etc., though. I'm fine with the S2 for a while.