I know of so many others who are trying to come up with a blog idea or theme which they hope to monetize someday. I always suggest that they consider a tips and tricks blog, much like this one, or Jsprik's weblog (who's keen on blogging about products and ideas). See, all of us grew up and know little bits of information that have been tried and tested that are also helpful.
I know of a site where visitors are encouraged to submit their own tips... but the traffic generated goes to that site. Might as well post what you know in your own blog, right? Who knows, you may even build on a following.
Start small. Blog about what you really know (money, real estate, weight loss, parenting) and expand on those later (with photography, advertising, etc.). Blog about your passions, or something your mother was passionate and had much to say about. You don't have to cover all the bases but you do have to be credible and cosnsitent.
Trust that if what you write are any good, readers will come.