In Netflix, there is an option to enable “smart downloads” which will automatically delete the downloaded version of a TV show once it’s been watched. It then queues another episode of that show for downloading.
For me at least, having an option like this in Plex would be very useful because I travel a lot. When I’m on a plane, I may watch three or four episodes of a TV show and I have to remember to queue the next episodes so they’ll download once I connect to WiFi again. It would be incredibly convenient if Plex would have the option to automatically do this and delete any episodes that have already been viewed.
You can also do this for TV Series by syncing the entire series or a season and selecting unwatched only and limiting the number of items. It will then sync that number of items and when items are watched then during the next synced watched items will be removed and replaced by the next unwatched items.