Well, a simple way could be through a portal. Imagine a magical portal opening between the mutant world and the wizarding world. Magneto steps through and is immediately in Hogwarts. The students and teachers are both fascinated and terrified. Harry Potter, being the brave one, approaches Magneto to find out his intentions. This encounter could spark a whole series of events where they either become friends or enemies depending on how Magneto reacts to the wizarding world's power structure.
In a more complex scenario, there could be a shared enemy. Let's say there's a powerful being that wants to control both the mutants and the wizards. This being has the ability to nullify magic and mutant powers. Harry Potter and Magneto, despite their differences, have to work together. They start by sharing their knowledge. Harry teaches Magneto about magic, and Magneto shows Harry how to use his willpower in a more direct and physical way. As they train together, they build a bond and a strategy to defeat this common foe.
Perhaps there could be a prophecy in both worlds. In the wizarding world, there's a prophecy about a 'strange power from another realm' coming to either save or destroy. In the mutant world, there's a similar prophecy about a 'powerful magic user'. These prophecies lead to Harry Potter and Magneto being drawn to each other. When they meet, they initially don't trust each other. But as they start to understand the true nature of the prophecies and the threats they face, they realize that they need to combine their forces. They then face numerous challenges together, from political opposition in both worlds to the actual battles against the forces that want to stop their alliance.
Chapter 273, ""Justice Will Eventually Triumph Over Evil", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. Early on, Jiang Li, who was reading a book, suddenly jumped in fright..
Chapter 348, "Zhu Ling in Dire Straits, Eight Camps Martial Contest, Betting Five Years—East Ridge Has Fallen!", is the clearest early answer because it turns Hong Yuan's situation into a concrete plot event. It opens with Even more fatal, the Inspector He Wuyang has since been colluding... and closes on Calculating for the year, the 60% tribute target equals 72 pounds of.... The key payoff is that the chapter closes with There was no choice; East Ridge's strength was just too formidable.
Chapter 417, "The Real Struggle for Survival has Just Begun", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter turns that into something concrete by moving from Yu Li glanced at Nina and took the crab leg, eating it slowly and...
Chapter 337, "Over Two Months Pregnant", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from He Hui's eyes widened with shock. to Let's all share the joy!" "Okay, Mrs..
Chapter 337, "Major Changes in Xia City, the Wealthiest Person in Great Xia", is the clearest early answer because it pushes Military Department into a new phase or setting. The chapter gives readers a real answer by opening on Great Xia Year 3, September 5th "Whoa... and ending on Lu Yang smiled and nodded, "It's no big secret, just ask your father.... In story terms, the chapter closes with The Minister Xia Chuan, the Seven Departments Director and Deputy Director;
Chapter 350, "Methods to Improve the Peak of Cold Resistance, Moao's 10th Town, Beishuo's Yang Fa, Nightmare, Lu Yun of East Ridge", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. The chapter turns that into something concrete by moving from Especially like the Nine Towns, not only surviving but also... to However, before he could take a closer look, a violent gust suddenly....
Chapter 489, "Has She Been Morally Kidnapped?", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event.
Chapter 89, "Brother He, how can you be so unreasonable too?!", is the clearest early answer because it turns Director Sun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Director Sun took a step back, "What's the rush?
Chapter 90, "Are You Even a Woman?", is the clearest early answer because it turns Woman's situation into a concrete plot event.
Chapter 382, "Signing Up for a Survival Reality Show, Are You Asking for Death?", is the clearest early answer because the violence around Sister Li becomes final instead of hypothetical.