This kind of defeats the purpose of having multiple federated communities. Politics on lemmy.world and politics on Beehaw are different communities with different rules and different people who can even see them. Some people are subscribed to individual communities because they like that community, not because they want to join in a free for all with the entire fediverse. They don’t want to go to lemmy.world because the first poster liked lemmy.world traffic or moderation better, they chose the community they subscribed to because they liked it better.
I think the better solution is a front-end collecting comments for a particular link in from all the communities you subscribe to. If you subscribe to three different politics subs and they all post the same link, then all the comments could be displayed at once, either interspersed (with some method of considering traffic when comparing vote totals) or in collapsible sections (effectively like a top level comment for each community).