One great way to generate a passive income is through affiliate marketing. Now, this does depend on the size of your list. Yes, size matters when it comes to your list. Especially if you're looking to make some serious money and do it on autopilot. But, list-building takes time. It doesn't happen overnight. And you need to add value to your list or you become obsolete.
If you have specialized knowledge in a certain topic, you can put together an online course to teach others. For example, if you have experience in real estate investing, you can create an online course “Real Estate Investing 101”. The benefit of an online course is that once you create the course material, you can sell it to as many people as you want.