Without giving raw access to the database wouldn't this be a feature of the platform and not the client?

The user could choose weights in specific categories, that the app could then take into account when building lists.

depends entirely on how much the server API can provide in terms of granularity. Hence allowing the clients to do more than now.

Nothing really stopping a mastodon client from hacking that together. Was thinking about this over the last couple weeks as well, even being able to specify a weight account you follow would be huge.

Just limiting high volume posters to some limited percentage of the feed would be cool.