Two things In need in a smartwatch: long battery life (at least 5 days), and copious music storage space. I mean, I also need a clock, sleep and heartrate sensors, step monitoring, etc, but they're stuff most normal smartwatches have already.

I'm currently using a Hauwei GT2 which I don't connect to the internet except when unavoidable, such as when transferring music from my phone to the watch, but I would gladly prefer an open source solution.