What if you let authors tag their newsletters with topics and then let readers browse by topic?

>I wonder what would be the ideal format for that?

Don't start with what might be the ideal format. Think of something, read comments here and in other places you announced this, consider them, and add a feature, then iterate on it based on experience and feedback.

But try to be somewhat canonical and orthogonal (not 100%). That often helps, IME. I sometimes see startups missing out on these principles, maybe because of the rush to get something out.

My 2c.

Take a look at medium.com. I think their homepage is too heavy with content, but is comparable to your biz model.