One idea could be that the Charmed Ones have to team up with the Winchesters to stop a powerful demon that is causing chaos in both their worlds. Maybe the demon has a spell that can break the magical protections of the Halliwell Manor and is also targeting hunters. Another could be a crossover where Phoebe's premonition power combines with Sam's psychic abilities to foresee a major apocalypse event that they must prevent together.
A great plot for a charmed supernatural crossover fanfiction might be that there is a new type of dark magic emerging that is related to the Mark of Cain. The Charmed Ones sense this strange energy and when they investigate, they cross paths with Dean. Together, they discover that an ancient witch placed a curse on the Mark of Cain long ago, and now they need to break it. This would involve the Charmed Ones' knowledge of spells and the Winchesters' hunting skills.
Chapter 415, "No Food, But Your Life", is the clearest early answer because it turns Guan Xiong's situation into a concrete plot event. The chapter turns that into something concrete by moving from Are you begging me for food now?
Chapter 504, "It's me, Gu Yu", is the clearest early answer because it turns Shen Ruobing's situation into a concrete plot event.
Chapter 710, "Handsome, Who Are You?", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. It opens with The man was wearing the white cotton attire of a beauty salon, the...
Chapter 379, "Is This the Way Huaxiu Treats Its Guests?", is the clearest early answer because it turns Is's situation into a concrete plot event.
Chapter 380, "From today on, you no longer need to work here", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Seeing the sudden change in the manager's attitude, everyone around....
Chapter 494, "Unrequited Love Turns Into Enmity", is the clearest early answer because it turns Brother Shaojing's situation into a concrete plot event. The chapter gives readers a real answer by opening on In reality, Su Yinwan had not noticed Bai Lu's presence at all.
Chapter 531, "She Really is a Remarkable Person", is the clearest early answer because it turns Crown Prince's situation into a concrete plot event.
Chapter 414, "The White Knife Goes In, The Red Knife Comes Out", is the clearest early answer because it turns Nina's situation into a concrete plot event. The setup is Third Master really lives up to his reputation;
Chapter 724, "He Is Not Minghai's Child", is the clearest early answer because it turns Rong Junsheng's situation into a concrete plot event. Early on, Jiang Yan's statement left everyone silent again.. By the end, Every time he encountered someone, it seemed as though he heard the....
Chapter 629, "The Family is Going to be Gone", is the clearest early answer because it turns Captain Ji's situation into a concrete plot event.