Your blog topic should be related to your niche and bring value to your potential readers. It is not wise to write about anything that comes to mind; you should write about your area of expertise.
Study the hottest trends on the blog search engines: browse the top blogs, the hottest blogosphere posts and comments, top tags, etc. Consider using the following blog search engines in your research:
Use this great guide to help you pick your theme and take your blog to new heights!