OTHER THAN HDMI 2.1 there's very little that the Apple TV needs.
HOWEVER... With the launch of Apple's gaming subscription, there is a huge argument that we'll see the A12Bionic chip put in it. No interest for me as i've never played a video game in my life!
What would you like adding to Apple TV? I guess for me, iTunes offering all of their catalogue in Dolby True HD would be very now! That's my only criticism of iTunes - the volume is so low on their films! If i'm watching on my Mac, you have to have the volume on max to get a hold decent level. It's strange as Apple likes to think that it's the best with everything and yet, it's seriously bad in this area!