What Apple could (and I wish they did) do is eliminate the traditional AV receiver. I recently purchased my first receiver and the setup was tedious. Trying to configure everything through a small LED screen is a joke.
"Apple TV AV" would use the connected TV act as it's interface. The AppleTV on steroids would have a GUI interface that would use logic to assist in the setup of the surround speaker, inputs, etc using an Apple remote or any iOS device via bluetooth or wi-fi. It would be intelligent enough to sense what's plugged in figure out the best way to channel audio. A dock on top would let you play through content and don't forget about AirPlay.
I'd pay $300-$500 for that piece of tech now.