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.
Hermione Granger and Batman would be great to focus on. Hermione is extremely intelligent and well - read in the magical lore. Batman is a brilliant detective with a vast knowledge of various sciences and combat skills. Their interaction could be really engaging. Hermione could introduce Batman to the world of magic, and Batman could show Hermione how to use her intelligence in a more practical, non - magical way in different situations. Also, Wonder Woman and Ginny Weasley could be an interesting pair. Wonder Woman with her Amazonian strength and Ginny with her powerful magic could team up and have some great adventures together, sharing their different perspectives on heroism.
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.
Definitely Alex Mercer. His powers are so unique compared to the Justice League members. He can shapeshift into anyone, has superhuman strength, and can regenerate from almost any injury. This would create a lot of interesting interactions with the League. For example, he could transform into Superman to test the limits of his own power against the real Superman, which would lead to a really cool showdown.