If you have absolutely no idea what the theme of your blog should be one suggestion is to look at several indexes of large portals on the Internet and see how they categorize themes and topics. Clickbank at www.clickbank.com provides a good example of what the main topics on earth are. However in general popular blog categories include news, society, culture, politics, religion, ecology, commentary, technology, business to business, art, information, recipes, web site building, humor, how-to, troubleshooting, rants and product reviews.
There are of course some topics that are more difficult to write about and that really do require some credentials or authority. For instance many people are tempted to write about politics, however the problem with writing this type of blog is that you can seriously turn off prospective buyers by maintaining certain opinions. The same goes for religious, medical or technology blogs. The most successful blogs are those who are written by someone who has some kind of expertise about a subject, otherwise you are just another weirdo on the Internet with an opinion.
The idea is to provide expert content on your blog; content that is...