There are multiple forks of Mastodon already being run on live servers, often ad-hoc forks with minor tweaks. If you define fork narrowly enough, even changing

There are multiple forks of Mastodon already being run on live servers, often ad-hoc forks with minor tweaks. If you define fork narrowly enough, even changing the character limit might count, and quite a few servers do that. Most forks do not break interoperability, which is very different from Bitcoin forks. Beyond that, Mastodon interoperates with other software that speaks ActivityPub like Pixelfed and Friendica.