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.
One idea could be that Ghost Rider somehow gets transported to the DC universe during a magical mishap. He teams up with Constantine to battle a demon that's threatening both the DC world and his own. Another plot could involve Ghost Rider's hellfire having an unexpected reaction with Kryptonite, creating a new type of weapon against Superman's enemies. Or perhaps he meets Raven and they combine their dark powers to fight an ancient evil that neither could defeat alone.
To balance the powers, in a Ghost Rider x DC Fanfiction, you could limit Ghost Rider's access to his full hellish powers at times. Maybe he's in a weakened state when he first enters the DC universe. For DC characters, you could give them a handicap based on the situation. For example, if it's Superman, make it so kryptonite is involved in the story in a way that levels the playing field.
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.
One interesting crossover could be Ghost Rider teaming up with Batman. Imagine the contrast between Batman's detective skills and Ghost Rider's supernatural powers. They could face a villain who has both a criminal underworld connection and a dark magic aspect. Another could be Ghost Rider and Wonder Woman. Her strength and his hellish powers would make for an epic battle against a common enemy like Ares, who might be corrupted further by demonic forces.