The kbin equivalent, which is baked in to the platform, is that communities would be able to define a set of hashtags that get cross posted into their community. So there wouldn’t need to be new communities.
The main issue though is how do you meaningfully cross post mastodon content to lemmy? Will we be able to see the replies from mastodon users? Will we be able to reply?
If lemmy users are happy to treat mastodon posts like any other external content, it could work well. But more than a bot would be necessary to fuse the two platforms.