I did this live stream awhile ago, but I finally got around to cleaning up the writing.
I wrote a SaaS Business Plan for my upcoming Get Git Me app: www.getgitme.com.
I go through some questions such as "What problems will this solve", "Who is my core audience?" and "How will I monetize"?
Hopefully if you're just starting up a SaaS business, maybe some of these questions will help you think through your own product?