Sounds like tags.
For example
“Post—philosophy, epistemology”
“Text”
Sounds like reinventing hashtags.
Discussion
If you do a post with a hashtag, your followers will still see it indiscriminately.
I understand that’s how it works now. But that concept would be the basic way to “fork” the feed in the manner described.
EX: I make posts on hashtags “music”, “philosophy”, “politics”, and “culture”. You (or some other hypothetical user) only cares for “politics” or “culture”, and so sets your feed to only show those hashtags and ignoring my other two tags.
An option to follow only a combination of npub+hashtag would work for me