In Jr. High, I took a great creative writing class. Every day, we were given a new topic to write about. I absolutely loved it, because I was allowed to open up my brain and write about anything that I wanted to.  We were given a time limit, so we could only write so much, enabling us to effectively handle writing under pressure. 

Each day, along with the topic being varied, the technical writing style would change. For instance, some days the topic would be "school" and the style would be "short story". One of my favorite styles was the "brainstorming tree".  This is more often seen in business settings when a group of people are trying to come up with presentation ideas. I think it works just the same for writing, if not better.

To start, you write out your topic, and draw a square around it. In this case, it would be the theme of the book you are writing. For demonstration purposes, I will assign the theme as pregnancy. From here, you would create lines from the center square, write out sub-themes, and draw circles around them.  A few sub-themes that could stem from this would be the characters that would be in the story, (possibly the women who are pregnant).  So, let's say you have six main characters, (women who are presumed pregnant). From here, you could add the people who got them pregnant, and so on and so forth. Each time you go to the next set of characters or events, you switch out from square/circle, to designate the context of where they fit into the story.

This process is actually quite fun! Once you begin adding activities, more characters, locations, etc. you will have a plethora of ideas. These ideas can easily turn into chapters, and eventually - a book!

Now, if this does seem a bit confusing for you, please break it down by category. Do this by creating one category for characters, one for locations, and so on. This may help keep things more clear. Another way you can do this is by chapter, focusing on one family at a time, breaking it apart, once you have the character 'tree' complete. 

However you decide to do it, have fun with it and be as creative as your mind allows you to. Remember, you are only brainstorming, at this point. Changes can always be made. No matter what, you will have plenty to write about.