Installing something new has been possible with Threema (for some) and Signal (for some others). But it has to absolutely nail down usability! Users leave an app they don’t understand in an instant. And TBH it starts with the Service, the Home Server and the App having 3 distinct names. Imagine this to be necessary for people using Whatsapp (app, server, protocol) or Telegram (app, server, protocol). When we really want to increase usage, we have to get better at marketing!
Just an idea, there are tons of rss clients for other fediverse services (e.g. Mastodon). Maybe it would be possible to fork one of those and replace the posting code with lemmy. Or, on a more meta note, write a bot that reposts and account from another service, and just use one of the many bot instances already existing.
Never seen one. But I can imagine some being useful. Bridge bots to autopost from other sources, translate bots or reference bots (like Wikipedias “Linky”), for example.