ActivityPub only goes so far, which is why you have such diversity in Fediverse applications.
However, I do want more consistent experiences over the Fediverse.
For example I haven’t yet found a way to view Mastodon posts on Lemmy (unless the posts mention a community), but the other way round is fairly easy.
Also I would like that client and server are more detached so I can use a Lemmy UI on a Mastodon instance or vice versa