I would like to see custom watch faces and custom complications. But that may involve implementing a theme engine and that would likely increase the code base quite a bit.
I don’t think a blood pressure monitor is going to happen because the watch band would have to include a bellows/air bladder and it would be bulky and prone to breakage.
As much as I enjoy Face ID on my phone, I just don’t see it working on the AW. Face ID would require making the display smaller or the bezel larger in order to include the dot projector, camera and sensors. So which would be acceptable; same case size with smaller display or same display size with larger case and bezels?
As much as I'd love to see custom watch faces I don't see this rolling out any time soon. Custom faces are reserved for partners like Hermes and Nike. What would be a nice alternative would be even more modular faces or a watch face "workshop" tab/app.
I'm sure in time manufacturers will find tricks to tuck the camera inside the screen or make it so tiny it can fit within the bezel.