One thing that I'd dearly like to see is a separation between equivalents of MUA and MTA. In e-mail, your mail client actually operates on emails and not some abstraction level placed on top of emails by the MTA. This lack of separation contributes to whole fedi sending around only Create Note messages, because people cannot create any other messages. It also prevents people from viewing APub objects of nonpublic posts, because Mastodon doesn't expose an API where it would proxy a query to a remote server for you, but once again places its own abstractions over it.
I think this wish has something to do with the way Mastodon is developed, and would be more likely to come true in a world you describe. (I've seen Mastodon PRs assume ~unchallenged that Mastodon will interact with Mastodon only (or things that will try to emulate Mastodon exactly).)
rysiek
oblomov