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 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.
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.
If we consider a crossover with the Game of Thrones world, Tyrion Lannister could be a great character to include. He's intelligent and cunning, and his interactions with House would be full of wit. And in a crossover with the X - Men universe, Charles Xavier could be included. His telepathic abilities could add a new dimension to understanding patients, and he could have some philosophical debates with House about the nature of the mind and medicine.
For a crossover with the Pirates of the Caribbean, Jack Sparrow would be an interesting character to include. He could bring his wily and unpredictable ways to Sherwood Forest. Maybe he's looking for some hidden treasure there, and Robin Hood initially mistakes him for an enemy but later they team up to fight a common foe.