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.