ok so, i think my ideal implimentation would be this: your instance acts as a post office.

your client addresses things, under the hood, to 'everywhere you havent blacklisted, and also where the instance has blacklisted' unless you specify it only to go to specific addressees (followers, mutuals, @)

this also would make it easyish to impliment local-timeline-only posts, or cluster similar-minded instances into 'herds'

blacklisting is ideal here because, if nothing else, the sheer ammount of CPU load whitelisting would cause if you wanted to federate to all of mastodon but nowhere else

@nire [citation needed], string comparisons are really cheap.