By introducing Yellow Lantern elements into Hogwarts. For instance, a Yellow Lantern could visit Hogwarts and start influencing students with the power of fear. This would create chaos and force the professors and Harry Potter and his friends to deal with this new threat.
Another way is to have a character from the Harry Potter world be chosen as a Yellow Lantern. They would then have to navigate between their loyalty to the wizarding world and the new power and ideology of the Yellow Lanterns. Their internal struggle could be a major part of the fanfiction, as well as how their new power affects the relationships with other characters in the Harry Potter series.
One idea could be a crossover where a Harry Potter character somehow gets a Yellow Lantern ring and has to struggle with the power of fear that comes with it. For example, if Neville Longbottom got the ring. He's a character who has overcome a lot of fear in the Harry Potter series, but the Yellow Lantern ring would test him in a new way.
It could be Naruto. His ability to use chakra and his many powerful jutsu like the Rasengan and the Sage Mode would give him an edge. He also has great determination and a never - give - up attitude. He could probably adapt to the magic in the Harry Potter world quite well and use it in combination with his ninja skills.
Doctor Strange would be a very powerful contender. His mastery over the mystic arts in the Avengers universe gives him abilities like time manipulation, astral projection, and creating portals. However, in the Harry Potter universe, Dumbledore is extremely powerful with his vast knowledge of magic, his ability to use powerful spells like the Patronus charm on a large scale, and his strategic mind. So it's really hard to say who would be the most powerful as they both have different strengths.
The two universes could be merged realistically by having a magical portal connect Gotham and the wizarding world. This portal could be the result of some dark magic experiment gone wrong or a new discovery by a powerful wizard. It would allow characters from both worlds to interact, which could lead to the situation of Bruce Wayne being involved in something like mpreg through some cross - universe magic.
Harry Potter himself could also be a very powerful character in the merged universe. He has his own magical skills and the experience of fighting dark wizards. He could teach the Stranger Things characters about the wizarding world and use his magic to protect them from the Upside Down threats. Also, his connection to Voldemort might give him some unique insights into defeating any dark forces that come from the merging of the two universes.
It would probably be Harry Potter himself. He has the power of magic, which is extremely versatile. He could use spells like 'Expelliarmus' to disarm enemies or 'Accio' to call weapons to his side from a distance in the midst of a Call of Duty - style battle.
The magic system in Harry Potter can be blended with Grimm's use of magic in its tales. In Grimm, magic is often more wild and unpredictable. In a fanfiction, Hogwarts could start to experience a shift in its magic due to an influx of Grimm - like magic. Spells could start to misfire in strange ways, like turning into animals from Grimm tales instead of the intended effect. This would force the students and teachers to adapt to this new, more chaotic magic, while also trying to keep Hogwarts safe from the threats that come with it, both from Voldemort's side and from the Grimm - inspired magical anomalies.
Well, you could start by having a portal or some sort of magical accident that links the two settings. For example, a misfired spell in Hogwarts could open a gateway to Nerima. Then, the characters from both worlds would start to interact. The Hogwarts students might be fascinated by the strange martial arts and curses in Ranma 1/2, while the Ranma 1/2 characters could be amazed by magic wands and spells.
Well, one way could be through a multiverse - spanning event. Maybe a powerful villain creates a rift that pulls Spiderman from his universe into the X - Men's universe where the Phoenix exists. Or vice versa, bringing a Phoenix - related character into Spiderman's New York.
You could also have a character who has a connection to both universes. Maybe a half - Naruto - half - Angeloid being. This character would act as a bridge between the two worlds. They would introduce the concepts of one universe to the other. For example, they could teach the ninja in Naruto's world about the advanced machinery of the Angeloid universe and at the same time show the Angeloid the importance of things like honor and loyalty in the ninja world. As this character moves between the two universes, the two start to blend together more and more.