To me, the biggest advantage of a federated tracker like this is the ability to self host without relying on any centralized service, so in no particular order
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A community maintained repository based database system, so when someone initiate an instance of your tracker, it starts off completely empty, and then the admin can pull from a "seasonal anime" repo for the tracker to track seasonal animes, an "anime movie" repo, a "j-drama" repo " a "Hollywood movies" repo or any combination of the above, or pull from an URL for a custom tracking list.
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Local/global average rating/rating distribution.
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Some kind of upvote system for reviews to discourage short, low effort reviews that plagued Letterbox. Lemmy's "active/hot" sorting algorithm is pretty good and you may be able to use just that.
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User created lists and also the ability to share them publicly or privately.
Ok, I assume you are familiar with uBlock Origin right? In uBlock, you can select from any combination of first and third party filter lists, or make custom filters, as long as they follow the same format. There is no one central authority that approves every filter list constantly, and the uBlock maintainer are only responsible for their own filter lists. Does that make sense?
Yes, like sharing a particular list only with certain people that you choose.