When i have no story Idea i look at a story based Challenge and follow them or twist it to my own liking.
Famous one is the not so berry Challenge
https://lilsimsie.tumblr.com/post/157671494755/not-so-berry-legacy-challenge
You could even pick one gen and just play with
You have the perfect life. White picket fence, loving spouse, beautiful children. But why do you still want more? You have a one time secret affair and will regret it for the rest of your life. Afterward you pour your soul into raising your children and fixing your marriage. You never admit the affair to anyone and dedicate your life to being the perfect mother.
Sounds interesting and every gen has a differente story and differente things that you will focus on, so you wont end up playing the same way.
Here is another link to more Challenges
https://sims-4-challenges.tumblr.com/post/176590126913/family-dynamics-challenge
Family Dynamics for an example
Being raised by a parent with an affinity for keeping close to home never quelled your sense of adventure and love for the outdoors when growing up. You love finding hidden treasures and collecting “useless” items, hoping to convince others of their value. Being made a single parent at a young age only helped to build the desire within you to remain free of the clutches of a “normal” job or relationship. And so, you devote your heart to running your own business and being the best parent that you can.
and even more...