If I were directing Apple's future watches department, I think I'd want two strands. First, and very obviously, try to do the things like BP, temperature, etc., which are accepted by the medical establishment as being useful measures.
And, in parallel, a team looking at what sensors exist, or could be developed, which could be incorporated despite there not yet being known or obvious medical utility. Is there a possibility of detecting very fine movements. Which might be able to show variations on muscle relaxation. And what could be detected by various lasers and how the underlying skin affects the reflected light.