Or, I think this can be, kinds of, archived by ‘simple’ enhancement at client side without changing today’s server side implementation. At Lemmy client, user can choose to group several different communities to user defined virtual community. Like today’s client, we have ‘All’ and ‘subscribrd’ 2 big groups, If the web/app client allow user define more group like ‘news’ ‘meme’ etc, and combined then in one view, then that’s pretty much meet your needs? Tbh, this will be very cool feature if any client can do this, or there is already such client?

No, you can run it on your own homelab just fine. If you don’t have it already, you just need a (usually free) dynamic subdomain so your instance have a normal URL instead of IP.