One way could be having a Hogwarts student discover a magical artifact related to dragons and getting transported to the world of 'How to Train Your Dragon'. Or, a dragon could accidentally fly into the Hogwarts grounds and cause chaos that both sets of characters have to deal with.
You could start by creating a magical portal that connects the two worlds. Then, have characters from each story interact and face challenges together. Maybe a dragon helps Harry in a Quidditch match!
Well, you could start by imagining a world where the dragons from 'How to Train Your Dragon' have special powers similar to Pokemon. Or, introduce Pokemon characters into the dragon-training universe and see how they interact.
You could start by imagining a crossover world where characters from both universes interact. Maybe have Hiccup and Tarzan team up to face a common enemy in a wild adventure.
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.
Well, one way could be to have characters from both universes cross over and interact. For example, a Jurassic World scientist discovers a hidden land of dragons and tries to study them. Or a dragon rider stumbles upon the Jurassic World park and has to deal with the dinosaur chaos.
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.
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.
You need to have a clear understanding of both universes. Know the characters, their personalities, and the rules of each world. Then find a compelling plot idea that combines elements from both seamlessly.
The 'Harry Potter Fire Lord' fanfiction can combine the two universes by creating a portal or a magical accident that transports characters from one world to the other. For instance, a spell gone wrong in Hogwarts could send a character to the world of the Fire Lord.