Oh, and no, BlueSky doesn't count as an alternative, because it's just as much a walled-garden again.

What is needed of an alternative is that it's open and free, and allows everyone to write their own software to communicate with everyone else. The ActivityPub standard (which underlies the Fediverse) is currently the best and most widely adopted we have, and it's crucial that the networks you use implement that standard properly, i.e. by allowing federation by default in principle, unless there's reason to block specific servers.