I have been working on newsletters for a while, but have finally coalesced around a theme: How the Web Works. I have always enjoyed explaining technical topics simply, which is the goal of this newsletter.
Check out the newsletter here: https://howthewebworks.substack.com
The content roadmap is as follows:

The idea is to start with what users know (open a browser, navigate to a URL) and then build knowledge of the web ecosystem from there.
While the newsletter is just on Substack for now, I have plans to eventually turn the content into a course to be sold (working out format ideas, but surely more than just written content).
While a Substack newsletter isn't exactly novel nor is the idea of teaching how the web works, I do have some great ways I have been getting subscribers.
Through invitations to my existing email list subscribers, links on my personal blog (https://typeofnan.dev), and links on my software developer interview guide (https://interviewguide.dev), I have been able to attract about 2,600 newsletter subscribers already! This makes me think the topic is resonating with folks (although I'm not sure how much people are willing to pay for it).
Some things I'd appreciate from folks here if you're interested: