Artemis would also be a great character to focus on. Her archery skills combined with Ghost Rider's hellfire could create some really cool combat scenes. Plus, her background and her own internal struggles could be contrasted with Ghost Rider's own tortured existence.
Definitely Kid Flash. He has a connection to both Young Justice and The Flash. His relationship with The Flash could be explored further, like how he feels about being in the shadow of the more famous speedster while also being a key member of Young Justice. And his powers could be a great asset in the crossover, as he can keep up with The Flash in terms of speed to a certain extent.
Definitely Subaru from 'Re:Zero'. His 'Return by Death' ability makes him a really unique character. In a crossover, seeing how he interacts with the super - powered heroes of 'Young Justice' would be fascinating. Also, Robin from 'Young Justice' could be interesting. His detective skills and leadership could contrast well with Subaru's more impulsive and desperate style of dealing with situations.
I think the Ghost Rider would be the most interesting. His otherworldly powers and his tortured soul make him a very unique character. In the Underworld setting, his hellfire and penance stare would be extremely effective against the vampires and lycans. Also, seeing how he interacts with the different factions in the Underworld, whether he's feared or respected, would be really cool to explore.
I think Superboy could be really interesting. He has that inner struggle with his identity and being part of the world, and Ghost Rider's dark and complex nature could add another layer to that. They could have some really intense scenes together as they both deal with their own forms of power and the consequences of it.
Miss Martian from Young Justice could be a great focus. Her powers of telepathy and shape - shifting make her a formidable opponent. Also, her experiences as a young hero trying to fit in and be accepted, especially when facing the more established Justice League, can lead to some really interesting character development and plot twists.
Superboy would be interesting as well. He's constantly trying to figure out his place in the world, and in a homefront fanfiction, he could be exploring his humanity further. He might get involved in community service activities to understand normal people better, which could lead to some really cool character development.
Definitely Superman and the Red Ranger. Superman is known for his incredible strength and moral compass, and the Red Ranger is often the leader of the Power Rangers. Their leadership styles could either clash or complement each other. Also, Batman and the Blue Ranger would be fascinating. Batman's detective skills and strategic planning could be contrasted with the Blue Ranger's more technical skills in handling the Ranger technology.
Definitely Robin. He's a bridge between the two groups already. He has his own experience in Young Justice and is also an important part of the Bat Family. His character development could be really interesting as he tries to balance the two worlds.
Ghost Rider and Batman would likely have a tense interaction. Batman would be immediately suspicious of Ghost Rider's powers and motives, while Ghost Rider might see Batman as a bit too controlling. Superman would probably approach Ghost Rider with an open mind, trying to understand his powers and how they can be used for good. Wonder Woman would respect Ghost Rider's warrior - like nature and his fight against evil, but might also be concerned about his more brutal methods. Flash would be excited to see someone with such unique speed abilities, and might try to race him. Aquaman would be cautious, especially if Ghost Rider's powers had any negative impact on the ocean or its creatures.
Batman would be really interesting. He's a master detective and strategist. He would be the one to study the Predator's behavior and try to find its weaknesses. His lack of superpowers but high intelligence would make for an interesting dynamic against the technologically advanced and physically powerful Predator.