I've been thinking this since the Apple Watch came out, but other than practical limitations (battery life) there's always this problem: have you tried to hold your arm out in front of your face and stare at your watch for a while? It's not the most comfortable position. You might have to add some extra arm/shoulder days in your exercise routine.
I think a fisheye lens plus face recognition should solve that nicely. As long as you can see the watch screen, the image could be cropped, skewed and rotated to focus on your face.
