For me, that advanced reader is Thunderbird. It has amazing customizability with userchrome.css, web extensions, about:config and developer tools from Firefox.
1. Convergent desktop/mobile app that syncs between devices (open source and local)
2. Performant for large numbers of feeds
3. Integrated browser and automagical discovery and organization of feeds while you browse
4. Multiple taxonomies and viewing layout options, chronological, by subject etc
5. Advanced filtering by keywords
6. Transparent and pluggable locally runing algorithms to track usage and inform the user of patterns and if desired adjust presentation
In a sense an advanced RSS 'reader' is what the web "browser" should have evolved to. There is really no real boundary between these two clients.
An advanced RSS reader is essentially a more dynamic browser that queries the internet in more ways that the user-initiated visit of a bookmarked url or typing something into a search form.
I'm using a combination of FreshRSS (self-hosted) and Readrops (for Android).
I feel like #1 is handled by this combination. But, I wonder what convergent means here for you?
I don't notice any performance issues. But, this feels like a simple task for a bunch of RSS feeds. Did you notice performance issues with other readers?
There is something very interesting about what you call "automagical discovery." To me this is the biggest hole in my RSS experience. I want something that gives me magical discovery within the feeds I have based on my reading experiences. I subscribe to a lot of feeds because of one article, and then don't care about 90% of the other articles, but there are 10% that I do really want to read, but don't have the focus to find them. Is that what you mean by automagical?
Why do you want #4?
Filtering by keywords seems interesting.
FreshRSS has a bunch of analytics that comes with the server. I never really use it, but it is interesting. I would be happy to share what I can see based on my limited usage over a few months if this would serve your purposes. I would be interested in understanding what you mean by that.
The one thing I wish I had in my current setup is a way to take notes in a centralized way. When I'm on Android, I suppose I can copy and use a share intent. And, on a browser, I could install an extension to do that. But, it feels like that is an interesting opportunity for someone. I wish FreshRSS could layer a JavaScript app on posts, for example, and then I could build whatever I want. It has an extension API, but strangely documented.
"Convergence" was a term used by Ubuntu in their brief foray into mobile Linux. Alas it never led anywhere, but it still embodies a sort-of holy grail of cross-platform apps with uniform native feel.
On performance, yes, in my experience various readers (have not tried them all!) will choke if the feed list grows beyond a hundred or so. If you make consistent use of RSS you don't even need to be a power user to reach this number. E.g. think of the typical total of browser bookmarks of youtube subscriptions people have.
On reader magic, yes, the empowerment of users in discovering relevant content was quite central in early visions of the web. The idea was that with metadata (linked data) and intelligent clients you will have a sort of decentralized version of the "you might also like" functionality that is now such a mainstay of centralized platforms.
Taxonomies and flexible layouts can really transform your experience. It gives you the option to switch between a reddit-like, subject-oriented view of feeds, or a mixed chronological order.
I am actually thinking of possibly trying out FreshRSS. Akregator is what I am currently using and ticks a few requirements [2] but does not seem to be having much further ambition (and being a native Qt application it requires serious commitment to contribute anything).
Yes your point about extensions is quite important. People have different ideas what makes "an advanced reader" and one way to reach broader community adoption is to make it easy to add functionality.
I've got FreshRSS installed. Email me at the address in my profile if you want to try it. Or, just create an account at reader.rss.surf on your own, I think I've enabled sign ups.
FreshRSS is interesting because you can customize it to use postgresql instead of SQLite (which I did). Normally each user gets their own unique SQLite database, but in my case it is split across postgresql and SQLite now. The way they structure their tables and databases is a bit strange across users.
