One idea could be that Batman discovers a new Kryptonian weapon that Superman unknowingly has a weakness to. This weapon was hidden on Earth long ago by a rogue Kryptonian. Batman then has to decide whether to use this knowledge to defeat Superman or find a way to help Superman overcome this potential threat together.
A young Kryptonian girl who was sent to Earth in a different pod than Superman. She has different powers and a different view of Earth. She could try to mediate between Batman and Superman, seeing the good in both of them and believing that they should work together instead of fighting. She might also bring new knowledge about Krypton that could be useful in dealing with threats.
Batman and Superman are obviously very popular. They are the main focus of the original movie, so in fanfiction, they often take center stage. Writers love to explore their complex relationship, whether it's a friendship, rivalry, or a combination of both.
A really cool plot could be a 'what if' scenario. What if Superman was raised in Gotham instead of Metropolis? How would his powers and values be different? And how would Batman react to having a super - powered being in his city from the start? In this fanfic, we could explore their relationship in a completely new light.
A plot in the fanfiction could be about Lois Lane being the key to preventing the fight. She discovers a secret that proves both Batman and Superman are being used in a political game, and she has to convince them to trust each other. In this way, it would add more depth to her character as more than just a damsel in distress. For example, she could use her journalistic skills to uncover the truth behind the forces trying to pit the two heroes against each other.
Sure, She-Hulk is related to Hulk. Jennifer Walters, aka She-Hulk, got her powers because of a blood transfusion from her cousin Bruce Banner, who is Hulk. This connection is a key part of their storylines in the comics.