In some fanfictions, she might die due to a betrayal within the team. Let's say a new member is secretly working for the enemy and they manage to set up a trap for Miss Martian. They could use some sort of kryptonite - like substance that affects Martians and she is unable to defend herself. Also, she could die in a mission where she's trying to rescue a captured teammate. The enemy might have set up a deadly defense system that she fails to bypass and she gets killed while trying to save her friend.
Superboy would be devastated. He has a special bond with Miss Martian, and her death would likely make him go into a rage. He might start lashing out at the enemy without thinking, blaming them for her death. Aqualad, as the leader, would feel a great sense of guilt. He would question his leadership and decisions that led to the situation. He might become more withdrawn and overly cautious in future missions.
First, do your research on the Young Justice universe, especially about Martians. Know their powers, culture, and how they interact with other characters in the show. Then, create a well - developed OC. Give them a unique personality, backstory, and goals. For example, your Martian OC could have a conflict between their Martian heritage and their new life on Earth.
Well, Tumblr can be a great place too. There are many Young Justice fanfic communities on Tumblr. You can search for tags like 'Young Justice Martian OC' and you'll likely find some great fanfictions shared by fans. Some fans also post links to their works on Twitter, so it's worth checking out the #YoungJustice hashtag as well.
One idea could be a story where Batman and Miss Martian team up to solve a mystery that involves both Gotham City and the alien world Miss Martian is from. Maybe there's a new villain who is stealing technology from both places.
In some scenarios, Robin might be facing a powerful supervillain alone and gets severely injured. For example, he could be up against a foe with enhanced strength and speed, like Deathstroke. The villain could have a well - laid - out trap that Robin stumbles into during a solo mission.
In a fanfiction, Robin could sacrifice himself to save the world from a major threat. He discovers a way to stop an alien invasion but it requires him to give up his own life. He willingly does it, knowing the importance of his sacrifice. His death shakes the foundation of the Young Justice world and his friends have to learn to move forward without him.
The team would be filled with guilt. They'd constantly think about what they could have done differently to save Robin. This guilt might lead to some members becoming more reckless in their attempts to make up for his loss.
They would be devastated. Some might go into a state of shock and denial at first. For instance, Robin might isolate himself to try and figure out if there was anything he could have done differently. Superboy could become extremely angry and start lashing out, blaming others or himself for not being able to protect Blue Beetle.