Perhaps the Marauders plan to help a young student who is being bullied. They could use their knowledge of the castle's secret passages to set up a series of events that expose the bully and protect the victim. They'd be using their skills for good rather than just for fun this time. They might also use magic to create distractions, like making a flock of magical birds fly through the corridors leading the bully away from the victim.
One possibility could be that the Marauders plan to prank the Slytherins in a big way. They might use their Animagus forms to sneak into the Slytherin common room and create chaos, like turning all their robes pink or making their beds float. It would be a classic Marauders' style of mischief.
In some fanfiction, the Marauders might plan to uncover a mystery in Hogwarts. For example, there could be rumors of a hidden chamber that has some powerful and dangerous magic item. They'd start by researching in the library, using their intelligence. Then, they'd use their map to explore areas of the castle that are off - limits. James could use his Invisibility Cloak, Sirius and Peter could transform into their Animagus forms to scout ahead, and Remus could use his knowledge of dark magic to figure out any protections around the chamber. It would be a dangerous but exciting adventure for them.
Perhaps Lily's plan in the fanfiction is related to uniting the wizarding community against dark forces. She could be planning to gather all the good witches and wizards, including those from different magical cultures, to form a powerful alliance. This would be a significant departure from the more individual - based protection in the original story.
If their plan was to protect Harry before he even got to Hogwarts, it could change a lot. For instance, if they managed to defeat or at least keep Voldemort at bay, Harry might grow up in a very different world. He could have a normal childhood without the threat of the Dark Lord constantly looming over him.
In such a fanfiction, one possibility is that the Marauders get to see how much Harry resembles James. They might be amazed by his courage which is similar to what they had in their Hogwarts days. Maybe they would offer him some unique magical advice based on their own wild adventures at school.
One possible plot idea could be that Harry Potter, using his own experiences with dark magic and defense against it, teaches the Marauders new spells and strategies. For example, he could introduce them to the Patronus Charm in a more in - depth way than they might have learned on their own. Maybe he also shares his insights on how to deal with various dark creatures, based on his encounters in the Forbidden Forest.
Well, perhaps in the fanfiction, Harry and Hermione teach the Marauders advanced magic that they themselves have learned in their own time. For example, Hermione could teach Remus some new spells for defense against the dark arts, and Harry could show James some Quidditch techniques that he developed later. They could also try to change the Marauders' view on Snape, trying to prevent some of the animosity that developed between them.
The plot might involve Harry and Ginny sharing stories of their battles against Voldemort with the Marauders. In the process of teaching, they could also learn from the Marauders' own pranks and their way of dealing with difficult situations at Hogwarts. This exchange of experiences could lead to new friendships and a better understanding between the two groups.
One possible plot could be that the Marauders, in some magical time - travel or alternate reality scenario, get to meet baby Harry. They might be overjoyed to see him and immediately vow to protect him. Sirius could be especially attached, given his future relationship with Harry. They might also play with baby Harry, making him giggle with their magic tricks.
A really creative escape plan in Harry Potter fanfiction could be centered around the Room of Requirement. The characters could enter the room and ask it to become a portal to a safe place. The room could transform into a magical gateway that leads them to a different dimension or a hidden wizarding community. Or they could ask the room to create a series of obstacles and decoys to distract their pursuers. While the enemies are dealing with the fake threats, Harry and his friends could quietly slip out through a secret exit that the room provides.
Well, it could be some new students who are very much like the original Marauders in terms of their mischievous nature and loyalty to their friends. Maybe a Gryffindor with a great sense of adventure, a Hufflepuff who is unexpectedly brave and loves to get into fun, a Ravenclaw with a brilliant mind for coming up with pranks, and a Slytherin who has a soft spot for his new group of friends.
Well, one idea could be exploring the marauders' earlier days at Hogwarts in more detail. Maybe showing how they formed their tight-knit group and came up with their mischievous plans.