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.
A different approach could be to create a new, hybrid setting. Maybe a part of the wizarding world that has been overrun by dark magic starts to resemble the dangerous and wild jungle of the Power Rangers Jungle Fury world. The characters from both sides would then have to work together to cleanse this area. The dark and mysterious elements of the wizarding world's dark magic would blend with the raw and untamed nature of the jungle setting, creating a unique and exciting backdrop for the fanfiction.
The settings can be merged by creating a sort of interdimensional hub. For example, Hogwarts could have a secret chamber that leads to Smallville. And from Smallville, there could be a strange rift that opens to the Devil May Cry world. In this way, characters can move between the settings. Another way could be through a magical spell in the Harry Potter world that misfires and creates a portal to both Smallville and the Devil May Cry world.
One idea could be a crossover where Harry Potter accidentally ends up in the world of Ranma 1/2. Maybe he's on a magical experiment gone wrong and lands in Nerima. He could use his magic to help Ranma deal with his curse, like creating a spell to keep him from changing into a girl when splashed with cold water.
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.
Sure. There is a lot of fanfiction out there that combines these three universes. For example, a common theme is the characters from different series teaming up to fight a common enemy. In some fanfictions, Sailor Moon might use her powers to help Harry Potter in his battle against Voldemort, while Ranma could bring his martial arts skills into play. It's really fascinating to see how different authors play with these crossovers.
Another approach is to have them meet in a parallel universe. In this universe, the rules of both the Star Wars and Harry Potter universes are blended. Harry might have a natural affinity for the Force because of this new world, and Anakin could discover he has a latent magical ability. They would then learn from each other and become a united front against any threats in this strange new place.
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.
Harry would be on guard at first. He has faced many dark threats in his life, and Madara would seem like another one. But as he gets to know Madara better, he might see some similarities in their struggles. Dumbledore would likely be very interested in Madara. He would try to understand Madara's motives and see if he could be an ally or a threat to the wizarding world. Dumbledore would be cautious but also open - minded in dealing with this new and strange character.
Maybe there's a time - travel aspect. A character from Infamous travels back in time to the era of Red Dead Redemption. They bring with them their knowledge of modern technology and superpowers, which completely disrupts the balance of the old - west. The character has to decide whether to use their abilities to change history or just survive in this new, dangerous world.
The desolate and harsh landscapes of God of War could be blended with the diverse regions of Westeros. For instance, the frozen wastelands where the White Walkers roam could be combined with the ice - covered areas in God of War. This would create a more extreme and dangerous environment for the characters.