The ability to assign a TMDB Movie ID to a TV episode.
My use case is for pro wrestling. Wrestling PPV/PLE events are considered movies by all metadata providers. To create a true wrestling library combining weekly TV shows and Events (in-order to create smart playlists), it requires manually creating a TV series for these events. Each episode corresponds to a "movie".
While this can be done manually, and this is how I've been handling things. Having the episode entries tied to an actual entry from a metadata provider would be way more convenient.
I understand this might be a niche use-case, I do think there might be alternative uses for a feature like this. Sometimes a longer film series has been broken down into a mini-series. Or fan-cuts.