I'm too new to post links, but if you click on the home icon and scroll down there's a section called The Sims 4 Stories and Legacies which has a bunch of challenges and legacies for you to check out.
My advice may be cliché, but get inspired by the story of a movie you liked or change it to your liking. But if I were you, I'd start with a couple with different traits. Goodie and Evilyn. Goodie is a villain and Evilyn is an Astronaut. Set Evilyn with the aspiration of "Friend of the World" and Goodie as "King of Mischief". Imagine Goodie lives a double life and Evilyn suspects that her husband is a villain. They have a son who will be called Goodevil, which will be the perfect blend of both. So... Wait! I liked this story! Let me play here...
I agree with @rosemow go and check out the thread see what challenge speaks to you and start your story from there. Or bbrainstorm story ideas and see which one is your favorite. I have two going on right now one is a ISBI where the founder and heirs all have a voice that talks to them because they are insane my other one is legacy story where a girl is left on an empty plot of land to fend for herself because she wont marry the man her dad wants her to. Good luck on coming up with your story. If you plan on sharing it i would love to read it. I like reading sims stories just as much as i like writing them.
Comments
http://forums.thesims.com/en_US/discussion/788042/list-of-challenges-legacy-and-other-for-sims-4
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase