I chat with a successful app dev one time long ago. His formula is rather simple. If it takes time to develop the app don't do it. In stead publish 10-20 apps in a day. Hire some people on the cheap to search and organize data. Even after a long time his idea what might be successful and what probably wont be is way off.

Ideas are like, an app that is just a list of some kind. Ill make up an example: An app that has searchable list of cities in Denmark with a typical picture for each. Here you could allow the user to write a note with each entry or allow sorting by distance from- but it is more effective to move on to the next "app". You make 20 country apps like that, non of them get any downloads except from one country that gets 10 per month/120 per year. That makes it a productive day.