You don't look for profitable markets, you look for groups of users. Groups of people within a certain segment will generate revenue; then it just depends on how you differentiate and how much of the searches on the App Store you can capture.

A random example: there are 1.6 billion muslims on the planet. Some of those will own smartphones. Most of those read the Quran. Meaning, some of those will look on the App Store for a Quran app. If you build a good Quran app, you'll be making money. It could be $50k a year, $100k or $500k; that I don't know. But you'll be making money.

Also, don't forget, even though my apps have never been featured, Apple does actively look for good apps. That can help significantly if you build something nice. But you still need to target a large enough group of people.