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 Emilia from 'Re:Zero' would be great to focus on. Her kind - hearted nature and her own complex backstory could bring a different dynamic to the 'Young Justice' world. On the 'Young Justice' side, Superboy would be interesting. His struggle with his identity and his super - strength could create some really engaging interactions with the 'Re:Zero' characters. Maybe he could learn something about self - acceptance from Emilia.
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.
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.
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.
One possible plot could be that Subaru from 'Re:Zero' somehow gets transported to the world of 'Young Justice'. He might initially be confused but then use his 'Return by Death' ability to his advantage. For example, he could help the Young Justice team in battles by being able to predict enemy moves after dying a few times. His unique perspective on life and death could also influence the team's moral decisions.
The interaction between Ainz and Roswaal would be fascinating. Ainz is used to being in a position of power and having complete control over his domain in Overlord. Roswaal, on the other hand, is a complex character in Re:Zero with his own motives and schemes. Their encounter could be a battle of wits as they try to outmaneuver each other, or they might find common ground in their pursuit of power and knowledge.
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.
Superboy is often a popular choice. His complex origin story, being a clone of Superman, gives a lot of room for exploration in fanfiction. Writers can delve into his identity issues and how he fits in with both the Young Justice team and in relation to the Justice League.