While most niches have the potential to be successful on Substack, some are definitely more conducive to paid subscriptions than others. Generally, most niche topics that writers discuss in their newsletters fall into two main categories: entertainment and education.
Entertaining content is great for growing a large list of free subscribers. Educational content is better for converting casual readers into paid subscribers.
The topic of your Substack newsletter should be as specific/well-defined as possible without excluding topics that you know you will cover.
As an example, personal finance is likely to broad of a niche. You want to pick a more specific topic within personal finance. This will both help you connect better with your audience as well as make you stand out from the competition.
Some example of potential Substack newsletter niches within personal finance include credit card churning/mile hacking, tips saving money as a single parent, ways to increase income as millennial trying to escape the 9-5 grind.
You niche can also be an interdisciplinary niche between two typically unrelated topics. For example, you could write about both money and music. This could be a newsletter on how the influx of venture capital is affecting the type of music promoted on streaming platforms.
If you are curious about what topics the top ranked Substack newsletters cover, you can check out the Substack leaderboard to see the top paid Substack newsletters and top free Substack newsletters.