Well, perhaps the Ghost Girl is a protector of an ancient artifact. The Young Justice team stumbles upon this artifact and she appears to them. She can't communicate directly but shows them visions of the past and future related to the artifact. They have to learn to trust her and work together to keep the artifact safe from villains who want to use its power for evil.
One idea could be that the Ghost Girl is actually a lost hero from another dimension. She has special powers that she doesn't fully understand and the Young Justice team has to help her control them while also trying to figure out how to send her back home. Another plot could involve the Ghost Girl being a spirit who is tied to a mystery that the team needs to solve. Maybe she was a victim of an old, forgotten experiment and her presence is a clue to stopping a new threat related to that experiment.
A possible plot is that the Ghost Girl was once a member of a previous version of the Young Justice team. She died under mysterious circumstances and now her spirit lingers. The current team discovers her existence and she starts to guide them, sharing her knowledge of past enemies and hidden secrets. However, there are also forces that want to keep her silent and the team has to fight to protect her and uncover the truth.
One idea could be that a new girl joins the team with a unique power. For example, she can manipulate time in small bursts. This causes chaos at first as the team tries to figure out how to use her power effectively. She might initially struggle to fit in with the group's dynamics, especially with the pre - existing relationships between the characters.
One idea could be that Ghost Rider somehow gets transported into the Young Justice universe. He might initially be seen as a threat by the team. But then they realize he's fighting against a common evil that has ties to the dark forces in both their worlds. For example, a demon that is trying to open a portal to bring more chaos.
One idea could be that Ghost Rider's powers are somehow needed to stop a demonic threat in the Young Justice universe. Maybe a powerful demon is causing chaos and the team realizes they need the unique abilities of Ghost Rider to send it back to hell. Another plot could involve a member of Young Justice being possessed and Ghost Rider having to help exorcise the demon while also teaching the team about the darker, supernatural side of their world.
Another plot could be a time - travel adventure. Maybe a powerful magic spell goes wrong and sends them back in time. There, they meet younger versions of the Justice League members. They have to figure out how to get back to their own time while also making sure they don't accidentally change the future in a bad way. This would involve a lot of action and also some interesting character interactions as they see the heroes they know in a different light.
One idea could be a crossover where the Young Justice team has to team up with the Justice League to face an interdimensional threat. Maybe a powerful villain from another dimension is trying to invade Earth and the combined skills and powers of both groups are needed to stop it.
Maybe a story where some of the younger members of the Young Justice team are frustrated with the Justice League's methods and decide to take on a mission on their own, but end up getting in over their heads and have to call in the League for help. This could lead to a lot of character development and some interesting dynamics between the two groups.
One idea could be a crossover where Ghost Rider teams up with the Young Justice team to fight a powerful demonic threat that neither could handle alone. Maybe they have to enter the underworld to stop a demon lord from opening a portal to Earth. Another idea is a story where a member of Young Justice is somehow cursed and starts to transform into a Ghost Rider - like figure, and they have to learn to control this new power while still being part of the team.
One idea could be that the Young Justice team gets accidentally transported to an alternate dimension where the Justice League has turned evil. The Young Justice members then have to figure out how to reverse the situation while also evading capture from their once - heroes. Another could be a time - travel plot where the Young Justice team travels back in time and teams up with the early days of the Justice League to stop a threat that was thought to be long gone but has resurfaced in their own time. A third idea is that a powerful new villain emerges who can only be defeated by a combined effort of the Young Justice and Justice League, but first they have to overcome their differences in fighting styles and leadership.
One idea could be a misunderstanding that leads to a battle. Maybe a villain manipulates the situation so that Young Justice thinks the Justice League has turned evil. For example, the villain uses mind - control technology on some of the League members and Young Justice has to fight to save them.
One idea could be a misunderstanding that leads to a fight. Maybe the Justice League mistakes some of Young Justice's actions as being too reckless and end up in a battle. Then they realize they were wrong when a real threat emerges and they have to work together.