You can’t have a default server unless someone is ready to pay for it. (Idk how Mastodon does it.)

What I’d do is:

1. have every instance list its most prevalent topics/communities/interests (technology, games, communism, memes…)

2. when the user is signing up, have them select their interests

3. try to find the ideal match. Let the user override if they want to, perhaps let them know if the community is tiny, requires approval etc, but other than that just show a “suggested instance: example.org, change link”