Bookmark Users
As far as I understand, there's currently no feature in Lemmy to follow or bookmark users. Creating bookmarks for other users just in the Eternity app would still be useful though. By that i just mean some list of user accounts, no custom feed of their posts. The Sync App currently has such a feature, called "Favourites".

View Instances
When visiting some community or some user account, I would like to be able to get to an overview of their instance, primarily, to see what kind of communities are on that instance, and to see the main feed of that instance. That would be particularly helpful for themed instances, like country-specific instances. The Connect App currently has this feature.
Being able to search for an instance or to search some instance for posts/communities/users might also be helpful to some but is less desired by me.

Overall, Eternity is still my preferred Lemmy App, particularly because of its UI, but some other apps have features I would enjoy here as well.