I’ve already went on on why merging communities is Bad for the Fediverse (and only really helps the big corpos that get into the Fediverse), so it’s good that the badness of that “solution” is acknowledged.
As for #2: multicommunities: I seem to recall Kbin already does that, so it should work. As for sub-issue 1, "To create a multi-community, you would have to know where each community is and add it to your list. ", well that’s what webrings are for! Let’s bring them back from the '90s. Basically get’s give the power of “static search” back to the users.
Numero 3 Electric Boogaloo: Making communities follow communities, is not much of a bad idea, but I’m wary fo the issues already mentioned in it. I’m mostly concerned also about it making it harder to maintain smaller Lemmy instances due to the extra communication overhead.
Sounds cute, but it has to be implemented without the current trend of Eurofaszionazism.
Whether you make a platform out of Mastodon, AP, Peertube, XMPP, whatever, they are no longer useful if you slap espionage, ChatControl, forced Israeli apologia or anysuch stupidity on them.