I’m constantly running into multiple groups that address the same subject. It would be nice if I could combine those so I saw all the posts in one place. For example, all the forums that are discussing UAPs it would be cool to just look at a single thing that combined them all. 
(Mlem dev here) Custom Feeds is something that’s on our radar. In an ideal world, this would be implemented on Lemmy’s side so that it’s available across all clients and not just Mlem. For this reason it’s not on our immediate agenda right now.
I believe this is closely related to how we manage numerous communities that focus on the same topic. It’s likely that many of these communities have little to no activity. Eventually, I suspect Lemmy will face a real problem with this given no cross community/instance admin role.