The Kingpin could be an interesting antagonist in a more street - level crossover. Imagine a crossover with Daredevil's world. The Kingpin, with his criminal empire, could try to use Ghost Rider's power for his own evil purposes. He might try to find a way to control Ghost Rider, not realizing the danger he's getting himself into.
Dormammu could also be a formidable antagonist. His dark magic and his desire to expand his dark dimension could put him in conflict with Ghost Rider. If there's a crossover with the Fantastic Four world, he could use his powers to try and take over Earth while Ghost Rider tries to stop him, and the Fantastic Four might get caught in the middle.
A powerful dark wizard from the human world who has found a way to cross over into Equestria. He wants to harness the magic of the Elements of Harmony to increase his own power. He sees the Ghost Rider as a threat to his plans and tries to eliminate him while also causing chaos in Equestria.
It could be Orochimaru. He is always seeking power and the strange and powerful abilities of the Ghost Rider might be something he desires. He could try to capture or study the Ghost Rider to gain an edge in his own plans for power and immortality.
A good antagonist could be a character from the 'Resident Evil' universe, like Albert Wesker. His superhuman abilities and evil plans for global domination would pose a huge threat to Alex Rider. Alex would have to use his wits and skills to stop Wesker's bio - terrorist plans.
An interesting antagonist for a crossover could also be an evil Kamen Rider from a parallel universe. Let's say there's a version of a well - known Kamen Rider who has turned to the dark side. This evil Rider could be jealous of Chalice's powers and wants to take them for himself. He could be forming alliances with other villains from different Kamen Rider series in order to achieve his goal, making him a formidable and complex antagonist in the crossover story.
It could be a powerful devil who sees the Ghost Rider as a threat to his plans in the school. Maybe this devil wants to take over the school or gain more power among the students, and the Ghost Rider's presence and his sense of justice stand in the way.
In a crossover with Snow White, the Evil Queen could be the main antagonist. She might be jealous of Tinkerbell's magic and try to capture her to use her pixie dust for her own dark purposes.
One great Vampire Knight and Ghost Rider crossover fanfic might be titled 'The Night Riders of Cross Academy'. In this fic, the dark and brooding world of Vampire Knight meets the hellish and vengeful world of Ghost Rider. The main character, perhaps Zero, could gain the powers of Ghost Rider, adding a whole new level of badassery to his fight against the vampires at Cross Academy. It could explore how his new powers affect his relationships with Yuki and the other characters.
In terms of anime characters, Ichigo Kurosaki from 'Bleach' could be an interesting addition. His ability to see and fight spirits could be related to Kiva's battles with Fangires. They could end up in each other's worlds and have to learn to use their powers together to fight off both Hollows and Fangires.
Another possibility is Kamen Rider Decade. Decade has the ability to transform into different Kamen Riders, and his presence in a crossover with Kickhopper could bring a lot of different elements into the story. He could use his powers to help Kickhopper access new forms or abilities, or they could travel through different Kamen Rider worlds together to stop a multiverse - level threat.
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.