Apple TV already essentially functions like this. It will be an AirPlay 2 target and it has Siri built in. They removed the optical port when the ATV4 came out, it won’t be coming back. Buy a $20 hdmi audio extractor if you want that functionality.1) Siri needs to become much smarter.
2) HomePod needs an aux-in.
3) An AppleTV should be able to act exactly as a HomePod (a future version should have audio outputs for connection to traditional speakers).