I feel like this is a big thing that Lemmy needs quite soon. Lemmy system makes things more fragmented than in reddit for instance, so it would be great to combat this a bit. Sooner rather than later.
Something like when you go to a community there would be a checkmark that would also include posts from a list of communities curated by the mods of the community (or creator). Then when subscribing you would have this list of communities with checkmarks next to them to and being asked “want to sub to these communities as well?” and the user can un-tick some if they want to. Then later they can add their own communities to it should they choose to.