What do you already know about that has some authority to it that could even be a possible tie-in with a website project you have?
If you just hoping for a fly-by-night method or niche to make quick cash vs the well-thought-out branding channels, then you never going to bank. Why play snakes and ladders, when you could just play ladders and ladders? lol
Make a list of all the channels you find interesting that you think "hey I could do that just better" its not hard to find those with things you are interested in or know much about.