FIRST QUESTION: WHAT WILL THE MYSTERY OR CRIME BE?
What will the mystery or crime in the novel be? Will it be a psychological thriller and mind control is the crime? Will it be a murder mystery? If so, what is the cause of death? If it's a science fiction plotline, what mystery will hook the reader and thread through the entire story only to be revealed at the end?
Any experienced author will tell you that any book, regardless of genre, needs at least a string of mystery or suspense running through it. This is what will keep the reader reading. What will the mystery, crime or element of suspense be in your storyline? The answer to this question will begin the workable plotline.
Be specific in your answer to this question: What specific crime or mystery will drive the plot? Is it a murder, a theft, a missing person, a haunted house, psychological event that changes a family forever, or something more unique? What are the simple circumstances that surround this mystery? Think broad strokes, just a list of three general details.
The type of crime or mystery will dictate the investigation methods and details. It will also dictate what clues will be needed in order to slowly reveal the mystery throughout the four-part plot structure.
The answer to this question may also reveal the complexity of the story. For example, a financial crime is more complex than a murder mystery. The choice of crime may also determine the book genre: if it's a grizzly murder, it may default into a noir crime. If it entails mind games or gaslighting, it may default into a psychological mystery. If it's a small town and there won't be any bloody details or romantic spice at all, it can be either a clean mystery crime novel or even a cozy mystery.
This question will also determine the core of your story, and may also determine not only a certain genre but a book category or two as well. Will it be a gritty murder mystery or a whimsical cozy mystery? Will it be a A Missing Person's novel that is all about the revelation of one secret after another? Will it be an amateur sleuth murder with a satirical edge?
If it will be a murder mystery, you will need to choose a cause of death. If it's a police procedural, you may need to cover fingerprints and DNA evidence. If it's a fraud crime, you may need to weave in a computer tech wizard to find the clues. If it's a private investigator novel, you may need to weave in surveillance, and neighbor interviews. Think about what avenues you as an author you will need to go down in order to tell the story.
A little tip for newbies: My first novel series was with a private investigator because it didn't require knowing police law or the intricacies of police training, etc. Taking on a police procedural, or an FBI agent plotline will require much more research and may not be the fodder for a first time author.
SECOND QUESTION: Who is the Protagonist or Hero of the story?
Is he/she a private detective, an amateur sleuth, a police officer, or maybe just a weekend visitor who is inadvertently led into solving a mystery?
Another tip is this: At this early stage in brainstorming, don't choose a background, or a motivation, or any personal traits or challenges for the protagonist. It's too early. You will need to coordinate and connect all the characters, the crime or mystery, and all the suspects and their motivations before you can weave any story around any of them.
In this phase of brainstorming, you are really choosing only what hat your main character will wear. But at the same time, you may be choosing the book genre and the ultimate marketing categories for the finished book -- which is a good thing.
THIRD QUESTION: Who is the Victim?
Determine the identity, background, and significance of the victim. This decision can affect the motive, the suspects, and the overall narrative story arc. The victim's characteristics can also influence how the crime impacts other characters or the community at large. Before figuring out who the suspects are, you need to know who the victim is and why they were murdered or wronged in some way. If it's not a crime story but just a mystery or thriller, you will still need a victim. They may not die in the story, but there will be at least an injustice done to them. What is that injustice and who is the victim of it?
The victim's background, their relationships, and their secrets are all vital. Were they likeable? Did they have known enemies? A compelling victim, even if flawed, gives the reader someone to care about and root for. Even if the victim hasn't been killed or murdered, who is being bullied or targeted as the victim and why?