I wonder whether some real time greeting chat room would be an effective solution that also expands on the social element.
My mind goes back to irc where often there would be 2 channels like foo-invites and foo itself. So, first you'd go introduce socialize etc as a social proof to join the main channel and the lobby channel would still have a lot of power users from the main channel to take in people not only admins.
Maybe that's something that would be worth exploring on Mastodon!