Hello Chaps!! I'm setting up 3 blogs. One in the internet marketing arena, one in the general health market and one in another niche market which I have yet to decide. The idea is to try and make them profitable authority sites that make money by allowing people to advertise on them. I can't decide on which niche market to base the final blog around. Your ideas are most welcome - remember, it has to be a subject that attracts a lot of advertising.
What gr8liverpoolfan said is true. In fact, you could combine sports and celebrities and probably make a blog just about "celebrity sport stars", or a mixture of both. For some reason, people flock to that kind of junk. I call it junk because its utterly useless information. :: shrug :: It works though and the profits are undeniable.
My advice would be to pick a niche that you something about so that you don't have yet another generalized blog about Forex, Celebrities, Finance, Insurance, etc., so that you know how really provide information that is original and know how to market to people who are interested in the same things that you are. Private advertising will require good traffic for anyone to even care, and it also will depend on your content. Just another generalized blog is a dime a dozen. Make your blog about something you know. It doesn't have to be one of the traditional "high keyword" blogs that everyone and their mother starts. If you know it well, and can actually contribute some passion and knowledge, it will be more successful than writing something that you don't know well, and it will probably be the one blog that has some longevity. Don't fall into the trap of following all the other trends and popular keyword blogs...there are thousands of them. Last piece of advice, Don't let Adsense dictate what you start a blog about. That is no way to start a business. There are many ways to make money online, adsense is not the best, or the highest paying.
There are so many good niches such as health,travel,real estate,finance,sports,technology,gaming etc. but i suggest you to go with niches on which you have at least some knowledge to share with others and capable to give best reads to your visitors. DON.