Rather than continuing to add sensor after sensor, I'd much rather Apple work on improving the interpretation of the data that the Watch already collects. For me, that includes getting more of the current data (more HR readings and more HRV readings) and then helping to add context to the data so that its actionable.
You can add all the sensors in the world and all the data, but if the data isn't correct, is too small of a sample, and/or isn't put into context, it's not that helpful. That's the next big step for wearables in general, I think. Context and interpretation.