A Harry Potter and Call of Duty crossover fanfiction might have the Hogwarts students being teleported into a Call of Duty battlefield. They would have to adapt their magic skills to the new environment. The Death Eaters could team up with the bad guys in Call of Duty. Wizards could use apparition to quickly move around the map, which would give a whole new dynamic to the typical Call of Duty gameplay.
Definitely Harry Potter himself. He's the hero in the Harry Potter series, so he would be a central figure in the crossover. He could use his skills and courage to lead the way in the new, combined world. Ron and Hermione would also be important, as they are Harry's closest friends and would support him throughout the adventure.
One idea could be Harry Potter characters being transported into the Call of Duty universe and having to use their magic in the middle of a modern warfare scenario. For example, Hermione might use spells to hack into enemy communication systems.
One interesting crossover could be the use of spells in the Call of Duty Zombies setting. For example, a wizard character from Harry Potter might use the 'Stupefy' spell to stun zombies instead of using a regular weapon. Another could be the Hogwarts castle being overrun by zombies, and the students and teachers having to fight them off using a combination of magic and the survival skills from Call of Duty. It could also be cool to see the Death Eaters team up with the zombies for some sort of dark alliance.
Perhaps a combination of Soap MacTavish from Call of Duty and Harry Potter himself. Soap is a highly skilled and experienced soldier, able to handle all kinds of combat situations. Harry has his unique magical abilities and his experience in fighting dark forces. Together, they could use their respective skills. Harry could use magic to deal with large groups of zombies at once, while Soap could use his combat skills to take out any remaining threats or handle any non - magical obstacles in their path.
It would probably be Harry Potter himself. He has the power of magic, which is extremely versatile. He could use spells like 'Expelliarmus' to disarm enemies or 'Accio' to call weapons to his side from a distance in the midst of a Call of Duty - style battle.
One idea could be a crossover with the 'Halo' universe. The soldiers from 'Call of Duty Ghosts' could team up with Spartans to fight a common alien threat. They could share their combat strategies and learn from each other's advanced technology.
One possible plot could be that Naruto and his friends are suddenly transported into the Call of Duty universe. They have to adapt to the modern warfare settings, with Naruto's ninja skills giving him an edge in close - quarters combat. They could team up with some of the Call of Duty characters to fight against a common enemy, like a rogue group that has stolen some powerful jutsu and is using it in the war - torn landscapes of the Call of Duty world.
Another option is a crossover with a post - apocalyptic game like Fallout. The Black Ops 2 characters could find themselves in the desolate wasteland of Fallout. They would have to adapt to the new environment, deal with radiation, and form alliances with the factions in that world. Their combat skills would be put to the test in a whole new way against mutated creatures and raider gangs.
The characters from Call of Duty, being mostly military - based, would likely be initially cautious and strategic when interacting with Mass Effect characters. For example, Captain Price might approach Shepard with a sense of respect but also wariness. He would be interested in Shepard's leadership skills and how they handle the diverse alien races. Meanwhile, Shepard would probably be curious about the more 'grounded' combat tactics of the Call of Duty soldiers.
I think the main protagonist of Black Ops 3 would be a great character to focus on. Their skills and backstory can be easily adapted to different crossover scenarios. Also, the specialists like Ruin or Seraph could bring a lot to the table. Ruin's brute strength and combat abilities would be interesting to see in a crossover with a more magical or fantasy - based universe. Seraph's high - tech weaponry and her own unique skills could make for some great interactions in a crossover with a cyberpunk - like world.