Tbh the best case would an open source fitness tracking watch. After some research the two hardest things are pulseox(even garmin watches are kinda inaccurate on ox) and battery life. I think its possible but most open source watches aim at the budget sector while a real fitness tracker sadly needs to be pretty expensive. It would be pretty cool if you had a bunch of sensors and you could just do anything with them(make custom workouts, sleep tracking, etc). If you think about it a smartwatch om hardware level is basically a nunch of sensors and the software just takes data, processes it and then spits out new data thats easier to understand.