An indie app developer is a single individual or small team of developers that are not part of a larger studio. Large development studios can afford to hire graphic designers, developers, project managers etc... Resource limitations can often hinder or stifle inspiration for indie developers.
One way to find inspiration for a game/app is to look at the past. What were your favorite things to do as a child? What are/were your favorite games to play? Take a look at the immensely popular Crossy Road game. They took Frogger and added some features, updated the graphics, and made it their own.
I am not saying create 500 clones of existing apps/games; however, you can use the basic concept and rethink it.
Of course after you build your app/game dont forget to come to ReviewHub to market it and get free reviews.