Lemmy needs to have the ability to assign both local and federated communities to specific user-defined lists so we can control how we consume similar content in the Fediverse. For example, I might be a Reddit refugee that's fond of the /r/patientgamers subreddit. But now i have many multiple *patientgamers communities across various Lemmy instances, Kbin, etc... I'd like to be able to create a /l/patientgamers list, assign all of the local or federated communities I want to that list, and then navigate to that list to consume all of the "patientgamers" content from across the fediverse in one feed. Mastodon has this, and it rocks.
I think the problem with this is that server load would balloon to a ridiculous degree. Everyone who runs an instance server now would need to upgrade significantly and the barrier to entry on making new instances would be high. Don't forget these are regular people mostly volunteering their time, effort, and resources unlike reddit which was corporate.