This is actually pretty close to some ideas I had for a microsub "middleware".
With the way microsub is built it would be possible to generate recommendations and category feeds without a centralized server. You can create a "client" that can automatically read channels from any server and create feeds / modify existing items.
I already have a decent working example in removing like posts from a feed and sending them as a daily rollup instead at https://glitch.com/~microsub-middleware
With this middleware idea there is also no need for specialized servers and clients, it should all just kind of work (hopefully)