Well, it could make James more protective of his remaining family and friends. He might become more serious and less carefree, as the loss of a sister is a huge blow. It could also fuel his determination to fight against whatever forces caused her death, like in the wizarding world, it might be dark wizards or some sort of magical curse.
It depends on how close they were. If they were very close, James might go into a deep depression. He could lose his motivation for a lot of things he used to enjoy. On the other hand, it could also make him stronger in a way. He might use the pain as a driving force to become a better wizard, to ensure that no one else he loves has to face such a tragic fate. He may also start to study more about the dark arts or dangerous magic in an attempt to understand what exactly happened to his sister and how to prevent it from happening again.
She might be a bit intimidated by them at first. Since James' friends are known for their mischief and popularity. But as time goes on, she could become friends with them too. For example, Sirius might take her under his wing like a little sister as well, teaching her some of his prank - pulling tricks.
Well, if James is a Death Eater in fanfiction, it completely alters the dynamic at Hogwarts. James would no longer be part of the group fighting against Voldemort. This would mean that many of the events that happened because of the Marauders' resistance to the Death Eaters wouldn't occur. For instance, the creation of the Marauder's Map might not have been for good reasons. And Lily's relationship with James would be much more complex. Maybe she was initially trying to save him from the dark side, but failed.
She might have been lured into a trap by a dark wizard who was after the Potter family. The dark wizard could have used her as a bait to get to James or his parents. Another way could be that she was suffering from a mysterious magical illness that couldn't be cured in time. This would add a sense of helplessness to the situation as everyone around her tried their best but still couldn't save her. It could also be that she got lost in a dangerous magical place like the Forbidden Forest and couldn't find her way out, ultimately meeting her end there.
If the device is a magic amplifier, it could greatly impact the wizarding world. It might make even the simplest spells extremely powerful. For example, a first - year student could perform advanced spells with ease, which would disrupt the traditional way of teaching magic at Hogwarts. The balance between different magical abilities would be thrown off, and wizards would have to reevaluate how they approach magic.
In some fanfics, the immediate aftermath might show a devastated Lily. She's left alone with a baby to protect and she's filled with grief and anger. Harry grows up in a home that's haunted by the memory of his father.
The main character is James Potter. But depending on the fanfiction, other characters can become important. For example, if the story focuses on James' role in the wizarding government, there could be Ministry of Magic officials as major characters. Or if it's about his personal growth, his son Harry Potter (in a different timeline perhaps) could be a key character, and they could have interactions that shape both of their characters in new ways.
One possible plot could be a heroic sacrifice. Maybe he's facing a powerful dark wizard or a dangerous magical creature. In the heat of the battle, he makes the ultimate choice to save his friends or family, and in doing so, loses his life. This would be a very bittersweet ending, full of both sorrow for his death but also pride in his selfless act.
His death could lead to a great sense of loss in the wizarding world. His family, especially his parents and siblings, would be devastated. It might also cause a shift in the balance of power. If he was seen as a future leader or a powerful wizard, his death could create a power vacuum that dark forces might try to fill.
One possible fanfiction could focus on the moments leading up to their death. It might show James trying to protect Lily and Harry until the very last moment. He could be shown using his wand to fend off Voldemort as long as he could, while Lily was huddled in a corner with baby Harry, praying for a miracle.
One great fanfiction about James Potter's little sister is 'The Potter Sibling Bond' on FanFiction.net. It delves deep into her relationship with James and how she copes with the family legacy. It has a really engaging plot that keeps you hooked from start to finish.