from most of the research I've done, and most of the successful people I've spoken to or heard speak, start by scratching your own itch. So, it doesn't have to be something you are an expert in, but just something that you know more than others. And, if you can find something that you would like to see or find, then maybe others are looking for that same thing.
So, I guess I'm leaning more toward finding a need or gap to fill, and spend some time getting better at it, or even if there is already some one that is the best at it, learn from them, and then try to repackage it for consumption.