If you're doing a crossover with the Mass Effect series, Shepard could be a great character to include. Shepard's leadership and combat experience against various alien races would be interesting when paired with the Horsemen's fight against the forces of Hell. Also, including some of the alien species from Mass Effect like the Krogan or the Asari could create some really cool interactions.
Definitely Dante from 'Devil May Cry'. Dante is a stylish and powerful demon hunter. His acrobatic combat style and his cocky attitude would contrast well with Death's more stoic and purpose - bound personality. They could end up in a situation where they have to fight a common enemy that threatens both their worlds.
In a 'Shadow of War Crossover Fanfiction' with 'Star Wars', Luke Skywalker would be a good character to include. His journey from a farm boy to a Jedi Knight could be contrasted with Talion's path. And Han Solo's roguish charm could add some humor to the story. If it was a crossover with 'Harry Potter', Hermione would be great. Her intelligence and knowledge of magic could interact in interesting ways with Talion's skills. Dumbledore could also be included for his wisdom and power.
In a crossover with Devil May Cry, Dante would be a great addition. His cocky and powerful attitude would contrast well with the more brute - force nature of the Splatterhouse protagonist. They could team up to fight some sort of demonic invasion that threatens both of their worlds.
John McClane from Die Hard would be a great addition. He has a similar tough - guy attitude and is good at handling high - pressure situations in urban environments. Another good character could be Ellen Ripley from Alien. Her experience with alien threats would contrast well with Rambo's combat skills. Also, Indiana Jones would be a perfect fit. His knowledge of archaeology and adventure - seeking nature combined with Rambo's combat expertise could lead to some exciting storylines.
Storm from the X - Men would be a great addition. She has control over the weather, which could create some really interesting scenarios when combined with Wolverine's combat skills. For example, she could create a lightning storm while Wolverine battles enemies in the rain, using the flashes of lightning to his advantage.
In a crossover with Hannah Montana, obviously Miley Stewart as Hannah Montana herself. She could be in Seattle for a secret concert and needs iCarly's help to promote it without revealing her identity. It would be fun to see Sam trying to keep Hannah's secret while also being her usual wild self around her.
In a crossover with the Lord of the Rings, Aragorn could be an interesting choice. He could bring his leadership skills and combat expertise to Los Santos. The GTA 5 characters might initially be skeptical of his noble ways, but they could learn from him. He could also help them fight against a new evil force that has emerged in the city, perhaps an ancient and powerful gang with dark magic - like abilities.
If it was a crossover with The Evil Within, Sebastian Castellanos would be a great character to include. His detective skills and his ability to handle the nightmarish creatures in his own game could be useful in the Outlast 2 world. He could team up with the Outlast 2 character to solve the mysteries behind the horror. Also, Pyramid Head from Silent Hill could be a terrifying addition. His presence alone would heighten the sense of dread in the Outlast 2 crossover story.
Definitely Goku. Their previous encounters in the Dragon Ball series make it a natural choice. Their power levels and different fighting styles would create great tension and action in the story.
Well, if you're doing a crossover with '101 Dalmatians', Cruella de Vil could be an interesting addition. She might see the Aristocats as a new target for her fur coats, creating a whole new conflict for the cats to face. And the Dalmatians could be allies of the Aristocats, helping them outwit Cruella.