look at the aTV apps that exist now, menus with simple artwork thrown in for good measure, and video playback. not much processing involved at all. just need to able to parse a RSS stream
for apple to open this up would be very easy. and most of the work for the developers would be on their servers.
to add games would take a bit more, they would have to add a new controller of some kind. either something like the roku remote, or using your iOS device, but if you're going to do this, why not just use the existing system and airplay from the iOS device.
so in my opinion...
the system for games is already in place, using airplay and another iOS device.
for video playback, like HBO GO, plex, or even time warner. the existing hardware can handle it, just need apple to open the software end of it.