Well, the story mainly focuses on the return of an old foe. Yugi and the gang are drawn into a series of events that lead them to discover more about the Dark Side of Dimensions. This includes new and powerful cards, and duels that test their skills like never before. There are also elements of mystery and a struggle to maintain balance in the world of Duel Monsters.
Yugi's friends like Joey, Téa, and Tristan could also be main characters. They have always been an important part of Yugi's life and the dueling story. In the epilogue, they might be involved in helping Yugi with some new challenges or they could have their own storylines related to the dueling world.
One possibility is that it delves deeper into the mysteries that were left unsolved in the movie. It could explore the origin of some of the powerful cards or the true nature of the dark forces that were hinted at. The fanfiction might also introduce new characters who are related to the existing ones, like a long - lost relative of Kaiba who has some secrets about the Millennium Items.
Obviously, Yugi and Yami are the most popular. They are the main characters and their relationship is central to the story. Writers often like to explore their bond further, whether it's in a more emotional or action - packed way.
One idea could be exploring an alternate ending where Yugi and Yami have to face a new threat in the Dark Side of Dimensions that was hidden all along. Another could be a story about their friendship growing even deeper during an adventure in that dimension. Maybe a fanfiction where they meet new characters who have unique powers and they have to team up with them to save both worlds.
Yugi is always an interesting character to focus on. After the events of 'Dark Side of Dimensions', he has a new level of maturity and experience. His relationship with the spirit of Atem has also changed, so exploring his thoughts and feelings about that would be great. Another interesting character could be Seto Kaiba. He's always been a complex character, and in the post - movie world, his drive for power and his rivalry with Yugi could take on new dimensions. And don't forget about the new characters introduced in the movie. They have a lot of potential for development in fanfiction.
A cool plot could be that there is a time - traveling aspect. A new villain travels back in time to change the events of 'Dark Side of Dimensions' in order to gain ultimate power. Yugi and his friends then have to follow the villain back in time and stop them. This would involve them having to navigate the past, deal with younger versions of themselves and their friends, and make sure that history doesn't get completely rewritten in a way that destroys their present world.
One of the most interesting plot twists could be Atem facing a new, more powerful enemy from the Dark Side of Dimensions that he thought was long gone. Maybe an ancient evil that was sealed away and is now unleashed due to some disruption in the spirit world. Another could be that his friends in the real world are in danger in ways he couldn't have predicted, like being possessed by dark entities from the other dimension.
Atem's character might develop by becoming more cautious. After all, the Dark Side of Dimensions is full of unknown threats. He could start to think more before taking action as he realizes the potential consequences of his actions on both his world and the spirit world.