The ability to create Collections with Media from Online Sources would be really useful. For example, I would be able to create a collection with Marvel movies that are available on Showmax and also add Marvel TV shows which are available on Netflix. I would also be able to add one season of a Marvel TV show that has already been downloaded offline.
This would truly allow Plex to become a Universal Watchlist / Streaming Service.
There’s a feature suggestion asking for an option to add items from Discover to your library which will also allow them to be added to a collection. I suppose you can comment/vote in that suggestion thread…