This would likely cause moderation issues across instances with similar subject matter but different rules. We don't necessarily need to share the load across instances like that. Simply federating as normal should suffice, and what we really want is a separate tool on a layer between the instance and user that can gather a lot of related instances and bundle them into one supercommunity.
I think this is very doable and would most likely be done in the form of a Lemmy/Kbin mobile app or website front-end. Something that doesn't dig its grubby mitts into the way the actual communities themselves are operated, but can integrate lots of them together into a convenient feed using some logical rules. Communities that want to be included in a service like this can place tags in their sidebars, in a standard configuration easily parseable by a bot, which can then easily aggregate content from appropriately tagged and crawlable communities. Communities without tags are either skipped, as it's a sign the owner either does not want to be included in the supercommunity or else has not done the requisite minimum to be considered worthy of inclusion, or else loosely bundled anyway using whatever half baked keyword matching a bored dev decides to cook up. Duplicate links in the feed can, at user option, be removed (leaving only the link to the post on the largest community/with the most engagement/on a selected default community) or posted anyway, giving an option of reducing spam or being easily able to engage with all communities.
Comments sections will be a little more difficult, but I'm a fan of a tabbed system where you can have separate sections to show comments from each instance that is commenting on a duplicate link.
Given the time and motivation I probably could eventually make this myself, but I have neither, so instead I talk about it here in the comments section and hope someone else takes up my torch. If anyone wants to actually use my stupid ideas, I give you free and full license to do so with or without crediting me for it. Go forth and build shit.