Character crossovers are a key way. Writers will bring characters from both universes into the same story. They might have Hermione meet Bella, and through their interactions, the two different worlds start to merge. The slash aspect can then be added by creating unique relationships between characters from the two series, like a budding romance between a wizard and a vampire. This not only combines the universes but also adds an interesting new dimension to the story.
Often through character crossovers. They put characters from both series in the same story. Like Harry and his friends could end up in Middle - earth, or the hobbits could find themselves at Hogwarts.
They can combine in terms of magic systems. In some fanfictions, the powerful and often incomprehensible magic in the Cthulhu universe might blend with the more structured magic of Harry Potter. For example, a spell in Harry Potter might be corrupted by Cthulhu's influence and become a more dangerous and unpredictable force.
Some writers create a situation where the problems in the Supernatural world are related to Hermione's world. Maybe a dark wizard has escaped into the Supernatural universe and is causing chaos. Hermione then has to team up with the Supernatural characters to stop this threat. This allows for a seamless combination of the two universes, as they both have to work towards a common goal using their respective skills and knowledge.
It might also combine them by setting the Tetris game in the Hogwarts castle. So, instead of a plain background, you have the Great Hall or the Gryffindor common room as the backdrop for the game. The sounds of the game could be replaced with magical noises like spells being cast or the flapping of a hippogriff's wings.
It can combine the two by having the characters cross over. For instance, Harry could be invited to MI6 to share his knowledge of dark magic as they face some sort of dark magical threat in the Muggle world.
Well, one way could be through a crossover spell or some sort of interdimensional rift. Maybe a character from Twilight somehow ends up in the Naruto world and has to adapt to the ninja way of life.
By having the characters cross over. For example, the Hogwarts students could attend a WWE event and get inspired to become wrestlers. Or WWE superstars could visit Hogwarts and learn about magic.
In a Transformers Beast Wars and Harry Potter fanfiction, you could have the Transformer beasts being hunted by the Ministry of Magic because they are seen as dangerous magical creatures. Or perhaps some of the Hogwarts professors are interested in the advanced technology of the Transformers. For instance, Professor McGonagall might study the shape - shifting abilities of the Beast Wars characters and try to find similarities with Animagi. Also, the battles between the good and bad sides in both universes could be intertwined. The Autobots could team up with the Order of the Phoenix to fight against the combined forces of the Decepticons and Death Eaters.
In many 'Master Chief RWBY' fanfics, they combine the universes by having Master Chief somehow end up in the RWBY world. This could be through a portal or some kind of interdimensional rift. Once he's there, his high - tech equipment and combat training from his own universe contrast with the more magical and combat - school - based RWBY world.
In such a fanfic, the writer might draw parallels between Dumbledore's wisdom and Yoda's Force insights. Maybe they create a situation where their powers and teachings complement each other in unexpected ways. There could also be a shared quest or mission that brings them together.