I would take a pet problem that I have and solve it well. Something so personal that nobody else would solve it. A personal time management app just for me. A way to manage all my contacts so that I can keep up with them intelligently. Something to get me to do that tasks I procrastinate on the worst (like going to the Dentist). There are a million general tools for this, but having your own perfect tool is like having a superpower. And it's a great way to get to know yourself.

Alternatively, you could become the world expert in something small and bizarre. Like VIM. Just kidding. No, like some API. I got to know the MTurk api, and it then became an expert at it. Then people started treating me like an expert. It was cool.

A third approach is find somebody you want to learn from and work with them. Any project you pick will never be as important as what you learn from the project.