Couldn't find this in the search here so not 100% it has been suggested before:

Can we have an new option to group communities into one feed? This was called a multireddit back on reddit, allows more granularity for feeds.

This is even more useful here on Lemmy, e.g. there are 3 separate communities on different servers about the same or very similar topics. A multicommunity of these 3 would simply merge their feeds into one. Then you won't need to check them individually for new posts anymore, just check the muticommunity.

Not sure how you would solve posting to a multicommunity, maybe needs an extra selection to which actual community this should be posted to.

I think there is already a Lemmy app that has implemented this, don't remember which one...