the whole reason why they are separate platforms is that they are for different use cases.

The reason they’re separate platforms is because they were developed by separate people/teams. They’re not even vastly different usecases. Link aggregation isn’t far from microblogging. The comment you made is even represented as a Note which is what all the microblogging fediverse platforms use for their posts. If you make a Note from a microblogging platform and include a link, that is represented differently from Lemmy’s posts (which is a Page I think?) but semantically they’re the same thing.

a Mastodon user would want person to person conversations, a Lemmy user would want topic-based discussion with less emphasis on who they are talking with, etc.

A conversation on Lemmy is started by a posted link, but again, there’s no real difference in this thread we’re participating in and a thread on a microblogging platform. In fact, microblogging users can participate in this thread. And this thread could have been started from a microblogging platform and it could be the exact same thread.

A fediverse app should display Actors (which could be a human user, Lemmy community, Kbin magazine, bot user, etc) and its list of posts and allow that actor to be followed (assuming the current user’s server supports following that actor type). The functionality is basically the same on every platform and the UX should be the same; the tricky part is showing/hiding features based on what the current server allows (e.g. if the app is displaying a user, it should show a follow button on microblogging platforms and kbin, but not on lemmy) and handling the side effects which are different for every platform