Character crossovers are essential. If a Red Lantern like Atrocitus were to enter the Potterverse, he could bring his unique view on justice and vengeance. This could contrast with Harry's more morally - complex view of fighting evil. Their interactions could drive a great story, as Harry tries to understand this new, more extreme way of dealing with wrongdoings.
Another way is through character development. A character in Harry Potter, say Neville, could go on a journey to master a bankai - like power. This could involve him training in a special place, like the Room of Requirement, which could transform into a training ground similar to the ones in 'Bleach' for bankai training. He would face challenges and grow as a character, all while blending the two concepts.
They can be blended by creating crossovers in the plot. For instance, the Shirai Ryu's weapons and fighting styles can be incorporated into the wizarding battles in Harry Potter. A wizard could use a Shirai Ryu sword enchanted with magic.
A good way is to focus on their shared traits. Both Harry and Daryl are survivors. Harry has survived Voldemort's attacks and Daryl has survived the zombie apocalypse. So, in fanfiction, they can bond over their experiences of facing tough odds. For instance, they could exchange stories of loss and how they overcame it.
Well, you can start by finding common traits. Both are young leaders in their own ways. Harry leads his friends at Hogwarts in their battles against Voldemort, and Ciel leads his servants in his fight against those who wronged him. So, a story could be built around them sharing leadership strategies.
To blend the two universes well, you could start by establishing some common ground. For example, both have elements of mystery and danger. You could have the magic from the Harry Potter universe interact with Slenderman's spooky abilities. Maybe Slenderman's presence distorts the magic around him in Hogwarts, causing strange things to happen. Another way is to introduce a crossover object or character that belongs to both worlds in a way.
One way to blend them could be to create a crossover universe where Naruto and Harry Potter characters exist in a world that also celebrates the Day of the Dead. For example, the magic in the Harry Potter world could be combined with the ninja techniques of Naruto. The Day of the Dead could be a time when the spirits of fallen ninja or wizards return. The characters might use their respective powers to communicate with these spirits or to solve some mystery related to the Day of the Dead. Maybe Naruto characters could learn about the magic of the Resurrection Stone from Harry Potter in the context of the Day of the Dead traditions.
The Dark Knight's gadgets could be combined with Harry Potter's magic. Imagine Batman's utility belt having some magical items from the Potter world. Or Harry using some of Batman's detective skills along with his spells. A fanfic could explore a scenario where Harry and Batman exchange knowledge. Harry teaches Batman about magic, and Batman shows Harry how to be a more effective fighter without relying solely on magic. This exchange could lead to new ways of fighting their respective enemies and create a unique and exciting story.
Well, you could start with a wizard who has lost his magic ability due to some dark curse. He then gets into an accident in the Muggle world. An ambulance is called and the paramedics try to save him. Meanwhile, his wizard friends are desperately looking for a way to restore his magic. The ambulance could be like a strange new environment for the wizard friends when they come to visit him, full of Muggle technology that they don't understand.
A simple way is to have Bruce Banner be a Muggleborn in the Harry Potter world. This would allow for his character to be integrated into the story more easily. The slash aspect could involve a relationship with Harry Potter himself. They could bond over their shared experiences of feeling different from others. Harry as a wizard with a unique past and Banner as a scientist with a big secret (the Hulk).
I don't know who the author of 'harry the red lantern fanfic' is. There are so many fanfic writers out there, and without more specific information, it's hard to determine the author.