A possible plotline is that Voldemort and Sauron form an alliance in the fanfiction. This would force the heroes from both universes, like Gandalf, Aragorn, Harry, Hermione, and Ron, to work together to stop this powerful union. It would be a grand adventure with battles and strategic planning as they try to outwit the combined forces of two of the most evil entities in their respective fictional worlds.
One interesting plotline could be characters from the two universes meeting by chance. For example, Frodo and Harry Potter might cross paths in a magical forest that exists between their two worlds. This could lead to them sharing their experiences and learning from each other's struggles against evil.
In this type of fanfiction, a plotline could be Harry having a second chance at life and realizing the power of his choices. He might choose to focus more on his studies and become an even more powerful wizard. He could also use his knowledge from his first life to form alliances that he didn't before. For instance, he might befriend some of the Slytherins who were on the fence about Voldemort. This would give him an edge in the fight against the Dark Lord and also change the dynamics of Hogwarts.
Gandalf is surely a popular character in such fanfiction. His wisdom and powerful magic would make him a central figure. He could be the one guiding the characters from both universes on how to deal with the new threats that emerge from the crossover. Hermione is also popular. Her intelligence and knowledge of magic could be very useful in this new combined world, and fans love to see her solve problems.
There are 'Second Chance' fanfictions where Harry has a second go at his fifth year at Hogwarts. In this scenario, he is more prepared to deal with Umbridge. He might also use his second chance to better train Dumbledore's Army from the start, making them a more effective force against the Dark Arts. Some fanfictions even have him getting a second chance to change the final battle, making it less costly in terms of lives lost.
Maybe in this fanfiction, Harry is given a second chance to save Sirius. He could figure out a way to reach Sirius before he goes to the Ministry of Magic. He might use some new spells or magical items he discovers in his second chance to break through the Ministry's defenses and rescue Sirius in time.
Maybe in this fanfiction, Harry's second chance at life allows him to prevent certain tragedies. For instance, he could figure out a way to save Sirius Black much earlier. He might have a premonition about what's going to happen and use his knowledge from his first life to change things. This would completely change the dynamic of the story as Sirius could play a more active role in Harry's life and in the fight against Voldemort.
One idea could be that a character who made a big mistake in the original story gets a second chance to fix it. For example, Peter Pettigrew could be given a second chance to not betray the Potters. Another plot could be a student who failed a crucial exam getting a redo and changing their academic fate. Maybe Neville could have a second chance to stand up to the bullies earlier in his Hogwarts years.
One interesting plot idea could be that a character who made a major mistake in the original story gets a second chance to redo a crucial event and changes the course of the wizarding world. For example, if Peter Pettigrew had a second chance, he might not have betrayed James and Lily, which would have a huge impact on Harry's life and the fight against Voldemort.
One interesting Harry Potter fanfic could be those that explore alternate universes, like 'The Marauders' Era Rewritten'. For LOTR, fanfics that delve deeper into the backstories of minor characters are great. For example, ones about the Ents before the events of the main story.
In 'Harry Potter second chance fanfic', a fascinating concept is a second chance for the House rivalries. Maybe Gryffindors and Slytherins getting a chance to understand each other better. For instance, Draco Malfoy could be given a second chance to not be so influenced by his family's dark views. This could lead to him becoming friends with Harry and his friends. Also, characters like Luna Lovegood could have a second chance to share her unique knowledge more widely, which might help in solving some of the mysteries in the wizarding world much sooner.
One idea could be that the One Ring somehow finds its way into the wizarding world. Maybe a Hogwarts student stumbles upon it during a field trip in an ancient forest. Another could be a wizard or witch from Harry Potter's world traveling to Middle - earth to study their different forms of magic. A third idea is that the characters from LOTR end up at Hogwarts to seek help in fighting Sauron, and they learn about magic wands and spells.