That is the idea behind a niche. Now how do we find that niche that will make us go viral? Truthfully it has a lot to do with luck. If you get lucky and hit on an idea that brings in hundreds of thousands of viewers a day you will be very wealthy. Take for example "myspace.com". No one would have thought to make a website where people can talk to others online the way you can with myspace, but the man that did just got lucky. He started myspace off just as a way to share your music that you have made. Then the community grew so large he started to allow anyone to make a page and put up a profile. Now myspace is what it is. A multi million dollar website.
Here is one great idea for how to think of a great niche. Think of a new way to get information to users. Every single human being natural wants to learn new information all the time. So the information will naturally drive them to your site. The part that you need to think of is how to make people interested and want to come back. There has to be a unique way to get this information across to the users.
It might sound fairly easy but it is not. There are very few people that make it viral. The best piece of advise I could give would be to be creative and think of new ideas. Don't be afraid to give them a try. You can win if you don't try.