Lemmy and Piefed need a "subcommunity" structure to get that ball rolling IMO.
EG you may have an overarching "sports" community, and in layers:
sports -> basketball -> NBA -> team
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So if your team sub was kinda dead, you could browse /c/NBA or /c/basketball and get a wider feed of other teams.
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Similarly, you could mark non-spammy posts to "filter up" to the higher communities that accept it. So you make a post in your team community, and folks browsing basketball might see it, depending on their settings.
Permissions would be granular to set this up, consensual for both the "upper" and "lower" communities.
I've written this up as a more general proposal ti help niche communities, but have yet to put it on the development gits as a feature request.