If you have iOS/Android development experience, you can look at the Developer sector, which is relatively niche IMO.
I've developed the iOS/iPadOS app, Proxyman for iOS [0], which gains steadily ~2k recurring revenue every month. Basically, it's an iOS app that helps you to capture and decrypt HTTP/HTTPS traffic on your phone. The app is needed if you need to inspect traffic from your app.
Because it's an iOS app, published on AppStore, you can provide a subscription pricing model: e.g. $4.99/month, $39.99/year, or lifetime $99.
I also develop the macOS version, but it's a huge market, which is out of context.