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.
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.
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.
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.
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.
A rogue military general could be a great antagonist. He has access to military resources and is using them for his own gain, like selling weapons to terrorist groups. He would be a tough opponent for the combined forces of Splinter Cell and Ghost Recon as he knows military strategies and has a lot of firepower.
A powerful dark wizard could be the main antagonist. He might be jealous of the dragon rider's powers and the bond with the dragon, and wants to break that bond and steal the rider's powers for himself.
It could be a powerful dragon hunter. This person would see Naruto's dragon as a prize to be captured or killed. They might have a personal vendetta against dragon riders in general.
Perhaps a powerful sorcerer who wants to capture the black horse for his own dark purposes.
For the Assassin's Creed side, Ezio Auditore could be a great main character. He is a very well - known and skilled Assassin. For the Ghost Rider part, Johnny Blaze would be an obvious choice. Their different skills and personalities would create an interesting dynamic. Ezio's stealth and combat skills combined with Johnny's supernatural powers would make for an exciting story.