Blood glucose would be amazing but seems iffy. Apple hired folks from a start-up that had a prototype "light" based reader but it didn't work in everyday conditions and took more power than you could likely fit into a watch. Might be too much to assume they'll have figured everything out for a version one iWatch.
HOWEVER...this speaks to the point I raised at yesterday morning's meeting about there being a watch that works cross platform with a fitness app you are currently using via your smartphone versus you having to start all over again with a new fitness tracker that is aligned with a particular watch.