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

1. 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.

2. Local/global average rating/rating distribution.

3. 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.

4. User created lists and also the ability to share them publicly or privately.

To me the coolest features would be

1. It tells me when new episodes come out
2. It recommends me new anime based on my history

Sounds cool good luck with the project! Imo scrapers are hard to get to work well or might still miss stuff. I would give people the chance to submit their own snow information, maybe limit it to their profile until moderation approval then put it out on the general list. Also I'm assuming you would have all the normal tracker stuff but I know the ability to log when I have watched something or rewatched something would be important to me. Like the actual dates. The SO and I like to review our show history and talke about it.

You can probably fork bookwyrm or another related platform. Might help you out to get started on the activity hub integration and functionality.