That is a another can of worms, unless you already have it thought out in your mind and if you can let the story play out and build in the text. If not, what have your thoughts been in the not so distant past? Where has your imagination been? I have many times considered what names will be in the story and picked the name of a person and thought about what actions might come from what that person dreamed of doing. Possibly asking people you know what they have always wanted to read but did not find the story to be read. Mystery can be depicted in many ways. Harvest the mystery and write your story around the mystery. Your job is to tell a story that will engross the imagination of people. Reading a mystery can help you determine what you would have written instead. Murder mystery as a genre does not require the complete story to be based entirely on the mystery as a plot. Include people and their lives and develop the stories within the story. Romance is a bit tougher, but it can push you down a different path from what you may have read. Every story is yours. Have fun writing and others will have fun reading your words. Pick a person in the past and give a fictitious accounting of what might have happened. Let it flow and let it grow.