One idea could be that the lycans and vampires in the Underworld are being haunted by some strange demonic presence. The Ghost Rider, sensing this unholy chaos, rides into their world. He could initially be seen as a threat by both sides but then they realize he's there to fight a common enemy, like a powerful demon that wants to use the eternal war between the lycans and vampires to its advantage. The Ghost Rider's hellfire powers could be a new and terrifying element in the Underworld's dark and violent setting.
I think the Ghost Rider would be the most interesting. His otherworldly powers and his tortured soul make him a very unique character. In the Underworld setting, his hellfire and penance stare would be extremely effective against the vampires and lycans. Also, seeing how he interacts with the different factions in the Underworld, whether he's feared or respected, would be really cool to explore.
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 a fanfiction, the Ghost Rider could form an alliance with Batman. Batman would be intrigued by the supernatural element that Ghost Rider brings. They could team up to take on villains who are using magic or other dark forces. Ghost Rider's powers of hellfire and his unearthly strength would be a great asset to the Dark Knight's strategic mind.
In a fanfiction crossover, Ghost Rider could end up in the Mystery Machine by accident. His flaming skull would surely freak out the gang. However, he might need their help to track down a villain who has escaped into the world that Scooby Doo and his friends inhabit. They could combine their skills, with Ghost Rider's supernatural powers and the gang's mystery - solving abilities.
A good Ghost Rider crossover fanfiction should have a well - developed plot. For example, if it's a crossover with Spider - Man, the story should find a logical reason for these two very different characters to meet. Maybe a new villain is causing chaos in New York and their paths cross while trying to stop it.
Another idea is that there's a curse in the Naruto world that has some similarities to what made Ghost Rider who he is. So, Ghost Rider comes to help Naruto understand and deal with this curse. They could go on a journey together, facing challenges along the way, and in the process, Naruto might learn some new techniques inspired by Ghost Rider's abilities.
In a Ghost Rider - Avengers crossover fanfiction, the story could start with Ghost Rider being drawn into the Avengers' world due to some strange rifts in the dimensions. The Avengers, being the protectors of the Earth, are a bit skeptical of this new, flaming - skulled hero. However, as they face a powerful interdimensional foe that can only be hurt by Ghost Rider's hellfire, they have to team up. Ghost Rider's unique abilities like his superhuman strength and his ability to track down evil could be crucial in the fight. Also, his relationship with the supernatural could open up new storylines for the Avengers who are more used to dealing with earthly threats.
Definitely Constantine. Constantine is a master of the occult in the DC universe, and Ghost Rider has his own connection to the supernatural. Their combined knowledge and powers could lead to some really cool adventures. They could face off against demons and dark magic users that neither could handle alone.