I think there may be a challenge or challenges that you haven’t pinned down yet. First is: what problem does this solve?
Second is, how will people know that they are housed under the same roof, so to speak? A small instance dedicated to NBA basketball may be interesting, but if it seems disconnected then people would be wary. Small specialty instances can be shut down without warning for all sort if reasons.A consortium of instances may help with this issue, as long as it is immediately clear through common branding that they are part if the same group.
Third is that different communities have different needs.
Following this thread because I think it’s one of the very interesting unanswered questions about federation.