Simple, if you want money make apps already exist and have many downloads.
Make app with user interaction, for example
Chats (Already exist, no problem important is money)
Simple Games (Already exist, no problem important is money)
Social Apps (Already exist, no problem important is money)
Photo Editor Apps (Already exist, no problem important is money)
Important is users, if you make your users back to your app later a time you made many money(Social App).
And really need lucky, you can make now a hard app, working 2 years + and get 50 - 100 downloads
And you can make a simple app and get 10000000 - 50000000 downloads.
I dont lost my time, my patience learning and making hard apps, and i m making money.