The BluRay/ HD-DVD is a very good choice, I too would agree to that idea. Also the screen on the front of the apple TV (Although, if USB were enabled then there could be a 3rd party product).
I think that a feature that would be good would be a parental feature, if you do have films or TV shows or whatever on your

TV then it would be cool to each have a USB dongle with a login key, which would be password protected for each home user which could be used on all machines to log on with. That way, if my friends come round & want their kids to watch a film, then they log in with there key & any films that are above their age then it doesn't show it? This could be used for music as well, where they can't listen to Explicit music. This would be easy to implement as all Apple needs to do is add in a tab on the "Get Info" section to add in the details on age restriction.
Another idea would be a device that could be used as a music server. Nothing to expensive though. The device could be remote logged in like your wireless router and controlled/setup via the web interface on your Mac. You could then add drives as you need & set it to different raid set's & alike. I know you could use a NAS, but if Apple created something inexpensive and with there history, it would be easy to use, like an iTunes Server app, but you could still have your own directory & use you USBpass to access it via any machine. With wireless becoming more stable for HD over the WiFi it could be suitable, but have this attached to your

TV's & Macs in your home & it would save having to have your Mac on at all times? So when you have a hard day at work, you can turn the

TV on, whilst that loads, pour a beer or a glass of your favourite wine then sit with the lights off & watch your favourite film or TV show, or, put your fav album on that you enjoy relaxing to to in front of the fire. Either way, put your feet & relax...
I think this would make the

TV enjoyable, this way you do not have to worry about having other computers on to enjoy your music, & no USB HDD. Only the DVD type drive.
The only other thing I would then add

, would be TrueHD audio out & the ability to play .vob files direct on my Mac & iMac. This way, films would be copied like for like, not compressed onto my file server. Then, run though my 1Gb network. Then the

TV would have to be 10/100/1000 base network compliant.
I know it seems a bit much, but I have a laptop with linux Ubuntu on it, & it plays .vob files direct from the HDD & only boots with my USB key which is too password needed too. I think with the security history of Apple, you can never be to carefull, especially as Apple becomes more popular & thus more prone to virus's.