Good question. Perhaps the technology isn't quite here yet at a price point that many people would pay. Perhaps the device's mission hasn't been defined well enough, either.
As for design, well, you could mimic big old square watches:
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Slight curves are a popular design idea these days:
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Or you could go super futuristic with an unusual shape:
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Nokia has a concept where it's a set of little screens linked together in a bracelet, and you can either twist your wrist to see the display of interest, or swipe to shift them all up a screen at a time.
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Some of those definitely work better. It took me a while to get used to just a Fitbit Flex, which is nothing compared to any of these watches. It could still use a more breathable band, but I often don't notice it.
Mission is definitely lacking. Samsung was nuts to make one that required a phone to use at all. Made me think of if Apple required cell service on an iPad instead of selling the WiFi models.
Just guessing at hardware, I can't imagine Apple making an iWatch that doesn't work with the iPhone 5 and up. Maybe $250 for a device that does all the little stuff this Fitbit does plus sort of AirPlays stuff from your phone. Like if my phone dings for email, I can look at my wrist to check it instead of finding the phone.
Oh, and the battery needs to last a week. This Fitbit lasts five days. Not bad, but mentally not as satisfying as a week. That would allow you to always charge on Sunday or something.