Thinking about how a fanfiction fediverse platform might look. Something that supports advanced search and multi-chapter navigation. I don't know if this defeats some purposes of defederation, but more robust federation of posts (fics) would be ideal since all the content is fiction (as opposed to the implications of readily federating people's personal microblog posts).
A fanart platform can be something like a booru. Would be nice to have a booru that required artist credit section to be filled.
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.