Other random ideas:

• Federation health monitor? Checks if posts are federating well.
• Activity summariser: eg shows how many posts/comments in selected communities since last visited.
  • EDIT: I’m thinking of Phanpy’s catch-up feature here (it’s a web app for microblogging platforms and easily one of the best social media clients there is … if you haven’t seen the catchup feature go check it out).
• link localiser? Probably better in the backend (as suggested already). But it intercepts links to posts/comments and provides links to the local versions.