Definitely Hellboy himself. He's the central character from his own universe and would be the main connection to the crossover. Then, Batman is a likely candidate. His detective skills and his willingness to work with 'outsiders' would make him a great fit. He could be intrigued by Hellboy's nature and abilities.
Homer Simpson is very likely to be involved. He's the main character and his bumbling yet loveable nature can create a lot of comical situations in crossovers. For instance, if it's a crossover with 'The Flintstones', Homer would probably be trying to out - eat Fred Flintstone at the local diner.
The villains from 'the real ghostbusters' like Gozer or Vigo could play a big part. In a crossover, they might team up with villains from the other franchise, creating a more powerful and menacing force for the Ghostbusters to face. And let's not forget Janine. She could be involved in some administrative or behind - the - scenes role in the crossover, providing support to the Ghostbusters while also interacting with new characters.
Naruto himself is very likely to be involved. He's the main character of his series and his unique abilities like the Rasengan and shadow clones would make for great interactions. Sasuke is also a top candidate. His Sharingan and his dark past could create interesting storylines when combined with hero concepts. Sakura, with her medical ninjutsu and strength, could also be involved, perhaps teaming up with a hero who has a similar healing - based power.
I think the Task Force members from Death Note could be involved in scp death note crossover fanfiction. They were always in the middle of the chaos caused by Light and the Death Note. In the SCP world, they would have to deal with not only Light but also the various SCP threats. Also, SCP - 105 could be an interesting addition. Her ability to interact with photographs could potentially be used in relation to the Death Note. Maybe she could take pictures of the names written in the Death Note and use it in some strange way.
Definitely Harry Potter himself. He's the central character in his own series, so it's likely he'd be involved in any crossover. Hermione Granger could also be a key character as she's smart and curious, and would be interested in learning about the 'Twilight' world. From 'Twilight', Edward Cullen is an obvious choice. His brooding nature and unique abilities would make for interesting interactions with the 'Harry Potter' characters. Also, Jacob Black could be involved, especially when it comes to the 'imprint' aspect as he has his own story arc related to that in 'Twilight'.
Naruto himself is the most obvious one. Since it's a Naruto - related fanfic, he would likely be at the center of any crossovers with god - like entities. His strong will and desire to protect his village make him a prime candidate for interacting with powerful gods.
Well, characters like Jiraiya could be really interesting in a 'naruto crossover godlike fanfic'. He has a lot of knowledge about the ninja world and different powers, so his encounter with god - like beings would be full of potential for interesting stories. Tsunade too, as a powerful Hokage, could be involved. She could represent the Naruto world's power structure when dealing with these otherworldly god - like entities. And then there's Orochimaru, who is always seeking more power, so he would be very likely to get involved in any situation where there are god - like powers to be had.
Well, obviously Naruto himself is the central character. Sasuke could be involved as he has a complex relationship with Naruto. In a crossover, if Naruto is betrayed and ends up in the Dragon Ball universe, Vegeta might be a character he interacts with. Vegeta could initially see Naruto as a weakling but later come to respect his skills. Another character could be Luffy from One Piece. If Naruto is betrayed and crosses over to the One Piece world, Luffy would likely befriend him and help him out, seeing Naruto's situation as similar to his own crewmates' struggles.
Batman and Kakashi would be likely to form an alliance. They are both very intelligent and strategic. Batman is known for his detective skills and Kakashi is a highly skilled ninja. They could work together to figure out the motives of the villains in the crossover and come up with a plan to stop them. They would also respect each other's combat abilities.
Definitely other Kamen Riders like Kamen Rider Kabuto. Their different powers and personalities would make for an exciting crossover. They could team up with Faiz to fight a common enemy or have a friendly rivalry.