I don't have plan to support customize format in RSS Brain for now. But personally I use feeds from RSSHub (https://rsshub.app/) a lot. It's very easy to add a customizable feed to RSSHub (https://docs.rsshub.app/en/joinus/quick-start.html#submit-ne...).
About the cutomizable params, it would be more complex since that means each person's articles need to have difference scores, which multiplies the storage and computation a lot. But on some level the priority of articles is customizable: you can provide customized "upvotes" for each of the articles in a customized RSS feed (an example to provide these fields in RSSHub: https://docs.rsshub.app/en/joinus/quick-start.html#submit-ne...), which will affect the final sorting of the articles.
One feature I would like to explore more is making algorithms completely customizable (as an advanced option perhaps). I.e. allowing changing all params manually in addition to being able to use your presets.