Wonderful you’re gaining traction here. I bet you can use Medium to grow a nice list.
As for your question, Oooooooohhhhh it’s a doozy. Much bigger question than email lists. Am I saying this to guilt you? To make you realize how friggin’ difficult it would be for me to answer that? Certainly. And how very dare you. But here are some thoughts:
1. You like to write about anything and everything, and I assume this refers to topics and subjects. But are there certain themes in your writing? Are there certain concepts, ideas, or overriding big-picture questions you return to? (See, if you ask hard questions, I get to too.) Answering this could help you hone your “brand” (ugh, obnoxious word) or just help you clarify what kind of writer you are. It could also help you identify a newsletter idea.
2. In all the pieces you’ve published, what’s gotten the biggest response from readers? Which stuff of yours gets read, shared, and commented on the most? It’s smart to lean into what’s working already. Try an experiment where you write more things that go deeper or wider on the best things you’ve written. This too could help you identify a “brand” (ugh that word again) or a newsletter concept.
3. While it’s true that writers with a clear focus and a tight brand often market themselves and grow more easily, ultimately, I say write whatever the hell you want. This is an issue for me as well, since I have varied interests. I don’t think any writer should struggle too much to constrain or contain themselves. Your readers should want to read you for you. Let all the flowers bloom.