I'd start with an informal reader survey. Talk to the "ideal" reader of your blog -- likely, you know these people personally. What questions do they have? What would they like answers to? What do they want to see more (or less) of?

Next, consider a formal reader survey. Michael Hyatt does a wonderful job and outlines his process here: http://michaelhyatt.com/tag/survey

I'm happy to help more. The answer to your question is right under your nose.