Basically a JustWatch alternative with more features...so needs to at least have those features
Have better watch list management...that thing can get large
API and data dumps for easy access to the information
Can't cost that much, not very many streaming services or movie services in the world. And the total number of movies ever made is in the hundreds of thousands not millions or billions. Even with TV, not more than millions surely. A single scraper and single Redis instance can do it.
Maybe allow people to run this themselves to avoid locale and legal restrictions.
Need to have movies that a user can say "I'd watch this if I could" and maybe aggregate those counts (see HN comment)