So, as somebody that hasn't used an anime tracker for anything other than a database to look up information about shows, is the intention to make something like Bookwyrm, but for anime? If so, then you might want to check out how they implement the population and federation of titles/reviews/comments/etc. I haven't used it before, so I am not sure how it all works.
As for populating your database of shows to begin with, the only anime database api I have interacted with before is AniList (this api is what @[email protected] uses). I just pulled up their api TOS and it does say this:
Prohibited from use within competing noncomplementary services of the same nature, including but not limited to Anime/Manga list/tracker services. Competing services may be authorized on request if they provide significant and sustained integration/syncing with the AniList API and AniList user accounts.
Something tells me they are not about to authorize a fediverse alternative to their own site, but you can always try.
I don't know all the legalities around things like image usage and whatnot, but I do know that Sonarr uses cover images and posters that it fetches from tvdb. Their api is also completely free to use for non-commercial purposes as far as I can tell.
As for feature requests, I would say that the most important thing (other than base functionality) would be to enable easy migration from the existing trackers. I am not a dev, but perhaps some way to migrate over existing reviews/watched status/etc. from MAL or AniList. Those sites have been around long enough that people have a lot of history saved up on them. They aren't about to manually re-enter all that again.
Good luck!