@mckaywrigley I have a bunch of questions I ask. What would you build for yourself? What did previous companies you've worked for need? What are you an expert in? What have you noticed recently that seemed broken? If you were going to build something just for fun, what would it be?
To the extent that we can give a brief answer to the question of where novel ideas come from, it's curiosity. That's what people are usually feeling before having them.