Christa B. answered 07/20/19
Even a Broken Clock is Right Twice A Day.
The simplest answer is to add scenes within your present storyline. If your shooting for a longer novel, you may need to introduce new characters to add new scenes for length. However that is not always necessary if you are content with the characters and storyline you have now.
A great way to brainstorm is through outline and character interviews. First, write down a summary of your story. When each character is introduced, the beginning and end of each climatic scene and each time the plot changes or twists. From there you can begin an outline and add or change things as you like.
As for the character interviews, it is a wonderful way to learn more about each character and their personalities. I like to write questions down that would come up in any normal interview such as favorite genre music, most embarrassing memory, etc. Then answer each question in the mindset of the character. From there you can add what you've learned into your storyline to help give it length and perspective.