Maybe Artemis's sister is an undercover agent in an enemy organization. She has to balance her loyalty to her sister and her mission. She could send coded messages to Artemis, and Artemis has to figure out what they mean. This would add a lot of mystery and tension to the fanfiction.
Another plot could be that Artemis's sister is a normal girl who gets caught up in the superhero world when she is kidnapped by a villain who wants to use her to get to Artemis. Artemis then has to go on a rescue mission with the help of her team, and during this process, their sisterly bond is tested and strengthened.
One idea could be that Robin and Artemis are on a solo mission together and get trapped in a strange, magical place. As they try to find a way out, their feelings for each other intensify. Another could be a post - battle scenario where they're both injured and have to rely on each other for care, leading to some romantic moments. Or perhaps a time - travel plot where they see alternate versions of themselves in different relationships and it makes them realize their feelings for one another.
One idea could be Naruto finding himself in the Young Justice universe and teaming up with Artemis. Maybe they go on a mission to stop a new villain who has the power to manipulate time. Naruto's unique jutsu could be a great asset in combat against this villain.
Maybe a story where Icicle Jr has a change of heart and tries to defect from his villainous group. Artemis is initially skeptical but decides to help him. Along the way, they have to evade capture from both the villains and some overzealous heroes who don't trust Icicle Jr's intentions. This could lead to some intense action scenes and character development for both of them.
A good plot idea might involve Artemis getting kidnapped. Green Arrow then has to track her down, using all his skills and resources. Along the way, he discovers some secrets about Artemis' past that she has been hiding from him. Or perhaps a plot where they are both framed for a crime they didn't commit. They have to work together to clear their names, while also dealing with other superheroes who think they are guilty.
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 plot idea could be that they are undercover on a mission as a couple. As they get into character, they end up having a real moment and kiss. This kiss then makes them question their true feelings for each other.
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.