Yep I'd like my own private bird dog algorithm to sniff out feeds that match my tastes. It could even run on my own desktop computer and not bother anyone else. Trading them like baseball cards does sound fun.
Regarding OP question about why Mastodon is still small:
1. Obvious network effects. Nobody's there so nobody's there. This will slowly improve. (But the people who *are* here are really interesting!)
2. Not a business so no marketing.