Please, not too many sensors. Long battery life and small size and low weight is more important. All the other smartphones and fitness watches have these sensors so get one of those instead.

Maybe a HW interface to attach other sensor modules could be an option, but could also easily become a distraction and pull down the overall experience.

[Edit] A wireless i/f (bluetooth, LP RF) is of course better to use as interface to any peripheral sensor modules.

A Pebble 2 HR with a colour screen would do the job for me, I think.

Never cared about Pebble's fitness/health tracking and never used it. I don't need my health reported to me in numbers every day, I'm not a performance athlete nor a hospital patient. Living my personal life trying to hit some numbers dictated to me by some app sounds like a horrifying idea, like a second work life at home.

I prefer accurate sensors over many sensors.

My last watch would measure my heart rate to be in the 140s while sleeping. I have since slept with a chest strap heart rate monitor and do not in fact have a medical condition.

Granted, that was a cheap watch, but I still don't see the use case of a smart watch for health tracking versus phone+100$ chest strap.

I believe the usefulness of these kinds of devices for health tracking is vastly overstated due to lack of accuracy.

Well I for one agree here. Some good sensors would be nice. Otherwise why even have a smart watch? Just get a regular. But there will be a tradeoff, there always is and we cannot have everything. At this point in time I will settle with something that is open and builds a healthy ecosystem.