The three major remaining are google news, apple news and reddit, all manually curated.
I don’t think there’s a lot of opportunity, users simply want free content, you can’t really monetize through ads unless you have a massive user base (ie user generated content), and acquiring content is really hard.
If you want to give it a try yourself, I recommend an algorithm à la reddit/hn and some basic clustering to avoid duplicates. You can get scores from twitter/fb instead of relying on your own users. The hard part imo remains scraping content and parse the text… well, and monetize of course.
I think one possible way to implement such a system would be to keep track of what people highlight and run some data analysis on that. Opt-in, of course, if you don't feel comfortable sharing that data.
I'm sure there's lots of great essays I've missed that are just waiting to be discovered again.