In a crossover with 'Attack on Titan', Eren Yeager could be an interesting addition. His determination and ability to transform into a Titan could be used in tandem with the Goblin Slayer's tactics. They could face off against goblins that have somehow made their way into the walls, or maybe even goblins that have been experimented on by the same people who created the Titans.
In a crossover with 'Fullmetal Alchemist', Edward Elric could be included. His alchemy skills could be used to create barriers or weapons to fight goblins. He could also be interested in the goblins' biology and try to find a way to stop their breeding, which is something the Goblin Slayer has been constantly fighting against.
If it's a crossover with 'Naruto', Naruto Uzumaki would be an interesting character to pair with the Goblin Slayer. Naruto's positive attitude and his powerful ninja skills could complement the Goblin Slayer's more serious and direct fighting style. They could work together to protect a village that is being attacked by goblins. Also, in a crossover with 'One Piece', Luffy could be paired with the Goblin Slayer. Luffy's determination and his ability to lead his crew could be useful when they are facing a large group of goblins.
In a crossover with 'Fullmetal Alchemist', Edward Elric could be an interesting addition. His knowledge of alchemy and his quest for the Philosopher's Stone could intersect with the Goblin Slayer's world in unexpected ways. From Goblin Slayer, Dwarf Shaman could be included. His knowledge of runes and magic could complement Edward's alchemical skills, and their interactions would be full of banter and learning.
Obviously, the Goblin Slayer himself is a must - he is the central figure. The Priestess is also great to include as she often accompanies the Goblin Slayer and provides support with her healing and purification magic. Her naivete and growth throughout the story can add depth to the fanfic.
There could be a cultural clash. The Doom Slayer comes from a more futuristic and hellish setting, while the Goblin Slayer is in a medieval - ish fantasy world. Their ways of communicating and understanding the world are different. However, this could also lead to some interesting exchanges. For example, the Goblin Slayer might be fascinated by the Doom Slayer's energy - based weapons, and the Doom Slayer could be intrigued by the magic and alchemy in the Goblin Slayer's world.
It could be a powerful demon lord from the Doom universe that has possessed a goblin king in the Goblin Slayer world. This combined entity would have the cunning and numbers of the goblins along with the raw power and dark magic of the demon lord. It would be a formidable foe for the Doom Slayer and Goblin Slayer to face.
Obviously, the Goblin Slayer himself would be one of the main characters. From Destiny, the Guardian, especially a Hunter with their stealth and agility, could be a great match. The Hunter's ability to quickly dispatch enemies and traverse difficult terrain could complement the Goblin Slayer's more brute - force approach when it comes to goblin - slaying.
In terms of long - range attacks, characters like Shikamaru in Naruto could use his shadow - binding jutsu to immobilize groups of goblins from a distance. The Goblin Slayer could then move in with his dagger and short - sword skills to dispatch them quickly. Also, Naruto's ability to sense chakra could be used to detect hidden goblin lairs or ambushes. The Goblin Slayer's knowledge of goblin behavior could then be used to plan the best way to attack these hidden locations.
The Goblin Slayer himself would definitely be one of the main characters. And perhaps Luke Skywalker from Star Wars could be another. Luke's Jedi skills and the Goblin Slayer's goblin - slaying expertise could create an interesting dynamic.
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.
One interesting aspect could be the contrast in their fighting styles. The Goblin Slayer is more methodical and strategic when dealing with goblins, using his knowledge of their habits. The Doom Slayer, on the other hand, is a force of pure destruction. In a crossover, perhaps they could learn from each other. The Goblin Slayer could pick up some of the Doom Slayer's more aggressive tactics, while the Doom Slayer might gain some strategic thinking from the Goblin Slayer.