In a fanfiction, the characters from 'Young Justice' could accidentally stumble into the realm of the 'Rise of the Guardians'. There, they are mistaken for enemies at first. But as they start to understand each other's motives, they realize they need to work together. For example, Superboy could learn a thing or two about controlling his powers from Jack Frost, while Jack Frost could learn about loyalty and teamwork from the Young Justice crew. This would be a great way to blend the two universes and explore different character relationships.
You could write a fanfiction where Tooth Fairy has to go on a solo mission to a far - off land to retrieve a special tooth that has the power to restore the health of all the fairies in her realm. Along the way, she discovers new things about herself and her powers.
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 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.
One interesting plot idea could be Hiccup befriending Jack Frost from Rise of the Guardians. Maybe they exchange their skills, like Hiccup teaching Jack about dragons and Jack showing Hiccup some cool ice tricks. Another could be that they team up to fight a common enemy that threatens both their worlds, like a dark force that can control ice and also enslave dragons.
One idea could be exploring Pitch Black's past before he became the villain. Maybe he had a family or a lost love that led him down his dark path.
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 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 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.
One idea could be a crossover between the two groups. Maybe the Young Justice team discovers a new threat that is related to the Runaways' past. They could team up to fight a common enemy, like a powerful villain who has been manipulating events from the shadows.
A great plot for Young Justice Invasion fanfiction might be to have the team deal with internal conflicts during the invasion. Maybe there are trust issues among some members because of secrets that come to light. For example, if a member was unknowingly related to one of the invaders. This would create drama and force the team to work through their issues while also fighting the external threat. Also, you could explore how the invasion affects the public perception of the young heroes. Are they seen as saviors or are people scared of them bringing more danger?