Ideas are creative
Pretend for a moment you are an artist. You are painting an apple, as a thousand artists have before you, yet you want it to be unique, new, say something that hasn't been said before.
You consider the big picture--what does the apple mean to you? You focus on the details--what shape is it? what color? You explore wild ideas--what color would the apple choose to be if it weren't red?
Creating a great market idea is like that. It's asking yourself, what am I missing? Because when you figure that out, you'll know where to look for your great idea.