I can just talk from my own experience. I have a mobile app in a very specific niche, that is used when people tune in their tv antennas. Typically gross about 100$/month took 5hours to build and I have not made an update in 3years. Maybe those types of incomes are to small for you but you gotta start somewhere. So my tip is mobile utility apps, they don't go out of style as quick. You wont get rich but its a nice additional income!