There's really no hardware features that have come out that the Apple TV would benefit from. It's already fast enough for what it does, can play 1080p video just fine, and is compact and uses hardly any power.
The last upgrade was to add 1080p support, so it was necessary. 4K TVs are still in their infancy and likely won't be mainstream for a few more years, so for the moment the current Apple TV suffices.
The last upgrade was to add 1080p support, so it was necessary. 4K TVs are still in their infancy and likely won't be mainstream for a few more years, so for the moment the current Apple TV suffices.