One interesting plotline could be a battle between two underrated Young Justice characters where they finally get to showcase their full powers. For example, a fight between Beast Boy and Terra in a death battle scenario where Terra has more control over her powers than in the original series. This could lead to a very different outcome and a whole new exploration of their relationship and abilities.
A cool plotline might be Starfire's origin story in the context of Young Justice fanfiction. How she ended up on Earth and her journey before joining the team. It could involve her past relationships on her home planet and how they influence her actions on Earth. Also, a plot where she discovers a new power within herself while training with the Young Justice group, which then becomes crucial in a big battle against a supervillain.
Perhaps there's a plotline where a new member joins the team during this 'in - heat' situation. They have to quickly adapt to the intense conditions and the team dynamics. The new member might have a power related to heat, like the ability to manipulate fire, but they first need to learn how to control it properly within the context of the team and the ongoing heat - related challenges.
One popular type of fanfiction in this regard might involve the Young Justice characters making bets on the outcomes of Death Battle match - ups. For example, Robin might bet Aqualad that a certain character will win in a Death Battle, and then there could be a whole sub - plot about how they handle the bet. Another could be where the Young Justice team studies the combatants in Death Battle to learn new fighting techniques. They could analyze how different characters use their powers and incorporate those strategies into their own training sessions.
Another plotline could be a death battle where a character from RWBY has to fight an alternate version of themselves. It would be a psychological and physical battle as they grapple with their own strengths and weaknesses in a different form.
One interesting plotline could be Robin trying to prove himself to Batman in a more complex and dangerous mission within the fanfiction world. Maybe there's a new villain in Gotham that only Robin can stop, but Batman is hesitant to let him take the lead at first.
One interesting plotline could be Robin and Superman teaming up to solve a mystery that involves both Gotham and Metropolis. Maybe a new villain is using technology from Gotham to target Metropolis and only their combined skills can stop it.
A possible plotline is that Wally's immortality is a curse rather than a blessing. He might see all his loved ones die over the years and struggle with loneliness. He could also be hunted by those who want to study or use his immortality for their own gains. In the Young Justice universe, this would make for some exciting and emotional story arcs as he tries to keep his secret and still be a part of the team.
Maybe in the fanfiction, Robin is misplaced in time rather than space. He could end up in the past, having to interact with younger versions of his teammates or even their predecessors, and has to use his wits to not disrupt the timeline while also finding his way back. This would involve a lot of character development as he sees his friends in a new light.
Some fanfictions might have a plotline where the team has to face an old enemy that has been resurrected or empowered by the events of Endgame. This could lead to intense battles and strategic planning. There could also be a story about the relationships between the characters evolving. For instance, romantic relationships might form or existing friendships could be tested as they all deal with the new world order after Endgame.
Perhaps a plotline where the fanfiction delves deeper into the backstories of some of the less - explored characters in the context of the 'Auld Acquaintance' arc. It could show how their past experiences shape their actions and decisions during this particular story arc.