It would be so useful to have the ability to sync content to the Apple TV (with Plex Pass, sure). It’s already implemented in iOS so it’s probably easier to expand the functionality to Apple TV. Apps such as VLC already support video storage and offline playback, so I guess it wouldn’t be an Apple policy violation.
There’s already an existing feature suggestion covering this topic. I suggest you vote/comment in that thread to avoid distracting the little attention this got so far.
Please use the search before posting duplicate suggestions (even if those or older threads). Therefore closing this thread as a duplicate.