Suggestion for Mastodon (to prevent eventual gMastodon instance): cap instance size at 100 - 1,000 account by default. Make it a social contract for instances not to federate with instances greater than that limit.
Or, a few years from now, say hello to “gMastodon”. (See history of email and the Web for a lesson in the future.)
That said, we might already be on that path.