The use of familiar colours, textures, shapes, and words could make the difference between success and failure, for instance. It’s up to you to figure out what familiar is to your audience.
Some people read a lot and are inspired to create something based on that. Other watch films, plays, television, sport, etc. And then another group of people observes life for inspiration. One thing all of these approaches have in common is that they require some interaction with the world. It’s more difficult to find ideas in isolation, though it’s not impossible.
Over time you may find a particular way of doing things to develop an idea into a book. That means you have become familiar with how you work and have turned it into a process with a clear result, whether it is positive or negative. Finding out that your idea isn’t going to make a good book is as valuable as finding your next bestseller.
If you are published the next thing to tackle is how to keep your publication in people’s minds for as long as possible which involves finding ways to extend your illustrations’ lives beyond the book and making plans to release images for maximum effect.