1) TV -> iPhone/iPad mirroring. I want to the TV screen to be mirrored on my phone so I can touch and swipe on elements from my phone, not look at the TV while swiping on the phone. I want to tap on the image of the movie, not swipe down and right to move a cursor around. I want to control the music/video playback while I'm in another room.
2) FaceTime support on AppleTV. As in I want it to be able to accept a usb camera so I can FaceTime the grandparents without holding an iPad up while sitting on the couch with the kids trying to get everyone in the picture.
3) Third party screensavers... with support for music visualizations like the Winamp Visualizer days. Or support for live web cameras, or simply playing videos like those 4k relaxation videos on YouTube.
4) Enforced standard video playback... The YouTube app is killing the video experience. Jumping ahead in a video is near impossible and getting closed captions to appear and disappear is a chore and a half.
5) Better music support. When playing music, it should give one tap access to the music controls, even if you are on the Home screen, in another app. Even getting to the controls from within the Music app is awful if you've navigated away.