I first started dabbling with Mastodon 5 years ago. In my country there only were 6 instances at the time. 3 were made up by anarcho-communists, 1 was dedicated to music, 1 was dedicated to video games and only 1 self-touted as a "generalist" instance.

Of course I made my first profile on the generalist one, but I quicky discover I couldn't post on the timeline of the music or videogames one. I could only reply to already existing discussions, which was fine but gave me a big sense of passivity. This is a real problem for which I now propose a solution.

If you think it's fine that the only people who can post in a specific community are the ones who have an account there (= giving all your data to the server admins) then maybe you have a completely different view of what should be possible on Mastodon.