The only way building in a camera works is if you position your AppleTV immediately center and just above or below your TV set. Anywhere else- which is where many put their AppleTVs- and the camera benefit starts suffering. For example, I store my AppleTV on a shelf maybe 8 feet to the right of the TV. If it had a FaceTime camera and I'm looking at who I'm talking to on the screen, they would be seeing me look far right of them.
And then there's the many who make their AppleTVs disappear BEHIND their TV or in a cabinet... for which a built-in camera could "see" nothing.
The best option along these lines would be to put a "normalized" USB port back on the box and let it have the capability to attach a USB camera that could be mounted on the TV. Then, people could have the box anywhere and place the camera in the ideal spot too.
Else, open up continuity camera so it can be a more permanent wireless solution instead of leaning on iDevices that tend to leave their household with their owners. Think of something like the ancient iSight camera perhaps powered by a USB jack on the TV or a USB block plugged in behind the TV somewhere.
I like the target of this idea myself... but there are much better implementations than building a camera into the box. That option would only work for a few who happen to place their AppleTV box visibly and ideally right above or right below the center of their TVs.