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.
Another possible antagonist could be the CCG in the 'Tokyo Ghoul' universe. If they were to somehow cross paths with the 'Death Note' world, they might see the power of the Death Note as a threat and try to take it. This would put them in direct conflict with both Kaneki and Light, as Light wouldn't want to lose his tool for 'justice' and Kaneki wouldn't want the CCG to gain more power to hunt ghouls. So, in this complex crossover situation, the CCG could end up being an antagonist.
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.
Voldemort would also be a key character. His obsession with immortality and his actions that led to many deaths in the series are fundamental to the theme of life and death. In a crossover, he could be pitted against new threats or forces that have different views on death, which would make for an interesting story.
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.
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.
Well, in scp death note crossover fanfiction, a cool concept is the clash of different power systems. The SCP entities have their own unique and often bizarre abilities, and when combined with the Death Note's ability to kill just by writing a name, it creates a really chaotic and interesting scenario. Maybe some SCPs would try to find a way to nullify the Death Note's power or use it to their own advantage in their never - ending pursuit of chaos or control.
Harry himself is very likely to be involved. Given his many encounters with death, from facing Voldemort to losing loved ones, he has a deep connection to the concept. He would be exploring the aspect of death perhaps to gain more power to protect his friends and the wizarding world.
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.