I think the most money will come from a micro-niche you create on your own. But are you trying to produce for networks or yourself? If yourself, take something you personally like and add that unique twist to it then market accordingly. Using innovation like that will land you some good sales
In general, use the tube sites to see whats popular - lets be honest, they can show us some good trends as well. xHamster is a good start.
Testing? Test your new ideas and content before dropping all your money into it. A few mini samplers dropped on the tube sites can do a test to see if it will be eaten up :D
Some Sunday ideas!