Well, Luffy would obviously be one of the main characters. Since it's about his memories, he's at the center of it all.
The Straw Hat crew members are likely to be important characters as well. They are an integral part of Luffy's life and their interactions with him would surely be part of his memories. For instance, Zoro's loyalty and the times they've fought together side by side would be in Luffy's memories. Nami, with her navigation skills and the times she's helped the crew avoid danger, would also be a key part of the story as it relates to Luffy's memories of their adventures.
They might gain a deeper understanding of his motivation and growth. For example, Zoro could see how Luffy's dreams have evolved over time, which would strengthen their bond even more.
One interesting concept could be exploring Luffy's forgotten memories. Maybe there are hidden traumas or past friendships that got lost in his adventures.
Usopp might be inspired to be braver. When he watches Luffy's memories, he could see how Luffy fearlessly faces powerful enemies. This would make Usopp want to overcome his own cowardice and be more like Luffy. And Robin, she would gain more insights into Luffy's kind heart. She might see how Luffy has always been inclusive and accepting of everyone, which would make her feel even more at home in the crew.
You could try some popular fanfic websites like FanFiction.net or Archive of Our Own. They usually have a wide range of One Piece fanfics, so there's a chance you'll find this one there.
Naruto himself is obviously a main character in these fanfics. His memories are the central focus after all.
Naruto and Ace are the obvious main characters. Naruto as the protagonist of his own world, and Ace whose memories are the central theme. However, it's possible that the fanfic also focuses on some of the antagonists from Naruto. For instance, Orochimaru might be interested in Ace's memories for his own evil purposes. This could lead to new conflicts and alliances within the story. Additionally, side characters like Shikamaru might be involved as they are often used to provide different perspectives and strategies in the Naruto series.
Well, it's Luffy and Hancock. Luffy is the energetic and somewhat clueless boy who is always up for an adventure. Hancock is the proud and somewhat reserved girl. In the high school fanfic, they are the ones whose relationship will develop, whether it's a friendship or something more romantic. Their different personalities will drive the story as they interact with each other and with other students in the high school.
They might be shocked. Luffy has so many wild and dangerous adventures in his memories. His friends would probably be in awe of how much he has endured and how strong his will is.
Anna is the central figure here as the whole concept revolves around her memory loss. Elsa is bound to be a main character too. Given their strong bond in the 'Frozen' story, Elsa would play a crucial role in dealing with Anna's situation. Kristoff, being Anna's love interest, would also be a key character. And Olaf, with his innocent and caring nature, would surely be there to try and cheer up Anna and assist in any way he can.
Since I haven't read it, it's hard to say for sure. But if it's a RWBY fanfic, it might feature the main characters from RWBY like Ruby, Weiss, Blake, and Yang. They could be the ones experiencing the space - related situations and the repeating crimson memories.