This is easy if you are a creative person.
Go through all the apps on Android and Apple, in subjects that interest you, and see what's missing.
Choose something that you would like to see, that would be of value to you. If you feel it's something needed then chances are other people will think it's needed too.
Then, go to a free-lance app coding web site and hire someone to create the app for you. Then upload it to Google Play and the App Store, and start making money!
