Well, it often starts with a premise that brings the two universes together. Maybe a dimensional rift or some kind of magical - technological event. Then the characters meet. For instance, Ghost Rider might be drawn to the chaos caused by the Transformers' battles on Earth.
In fanfiction, the crossover can work through shared themes. Both Ghost Rider and Transformers deal with concepts of power, good vs. evil, and protecting the world. So a writer can play on these similarities. For example, the Autobots and Ghost Rider could fight side by side against a common enemy that threatens both their existences. The Autobots' technology and Ghost Rider's supernatural powers would complement each other in the fight.
The Ghost Rider in Star Wars fanfiction might be a result of a strange experiment by the Empire or some rogue scientists. They could have been trying to create a super - soldier or a new form of weapon. But instead, they accidentally summoned the spirit of vengeance. This Ghost Rider could then go on a rampage, targeting the corrupt elements within the Empire, and in the process, draw the attention of the Rebellion and the Jedi, who would be both intrigued and cautious about this new and dangerous being.
Well, one possibility could be that Ghost Rider's hellfire powers somehow interact with the Transformers' technology. Maybe his fire could be used to supercharge a Transformer's weapon or power source. For example, if Optimus Prime were to team up with Ghost Rider, Prime could use the enhanced power to defeat a more powerful Decepticon.
For the Ghost Rider universe, it could introduce advanced technology. Imagine Ghost Rider using a Transformer's weapon against his demonic foes. It would change the power dynamics in his battles.
In the world of 'Bleach', Ghost Rider's powers would be very different. His chain could potentially be used to bind Hollows or even Soul Reapers if he were to be an antagonist for a while. His Penance Stare might have an interesting effect on Hollows, perhaps forcing them to relive all the sins they committed while alive. However, it might not work on Soul Reapers in the same way as they have different moral compasses.
Well, in this crossover, the Transformers' world and the world of Esther: The Girl Who Would Be Queen merge. There are shared themes and conflicts that drive the story forward, and it's a fun exploration of how these different elements can come together.
In this kind of fanfiction, the Transformers could also bring new elements to Naruto's world. Their advanced technology could be something that the ninja villages are either in awe of or try to study. The Autobots could potentially form an alliance with the ninja, teaching them about their technology in exchange for help in their fight against the Decepticons. Naruto, being Godlike, could be the key figure in bridging the two worlds together, using his unique abilities to communicate and work with both the ninja and the Transformers.
In a Xenomorph Fem Harry fanfiction, the female Harry can be a symbol of change in both universes. In the Harry Potter world, being female might make her face different challenges and prejudices, which she can then overcome. With the Xenomorphs, her femininity could be something that intrigues them or makes her stand out. She might use her female intuition to figure out the Xenomorphs' behavior and motives, which could be crucial in any sort of alliance or conflict situation between her and the Xenomorphs.
Another concept is that Naruto is possessed by a spirit similar to the Ghost Rider. His friends initially fear him, but he has to prove that he can use this new power for good. He might have to learn how to use the power without being consumed by the rage and darkness that often comes with it.
There's a Ghost Rider crossover with Blade that's really interesting. In this fanfiction, their shared vampire - hunting nature brings them together. They have different approaches though. Ghost Rider with his hell - based powers and Blade with his vampire - slaying skills make for an exciting dynamic as they face a new breed of vampires that are immune to normal vampire - killing methods.
In Young Justice fanfiction, if Ghost Rider were to be introduced, it would change the dynamics completely. His presence would likely draw out the villains who are interested in his power or are afraid of it. The Young Justice team would have to learn to work with someone who has a very different moral code and a power source that is based on vengeance. This could lead to some really intense storylines where the team has to decide how far they are willing to go to protect the world with the help of Ghost Rider.