My suggestion is to be able to create a collection from within plex that can search the internet for artwork and metadata.
Example if you name the collection Harry Potter or Harry Potter Collection then have a box underneath it to tick which says “Search the internet for artwork and metadata”
There’s already an existing feature suggestion discussing an option to match manually created collections. I suggest you comment/vote in that thread in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.
Having read through that thread now, no real solution to the request I see, that seems be specific to tmdb whereas mine is not specific to any movie or tv show database site.