She might meet the Red Queen. They could have an intense conversation where the Red Queen tries to assert her authority over Alice's mom, but Alice's mom stands her ground. Or she could encounter the White Rabbit, who is always in a hurry and might accidentally lead her into some unexpected adventures.
Alice's mom could come across a whole group of new and original characters created specifically for the fanfiction. For example, a friendly fairy who guides her through the dark forests of Underland. She might also meet a lost knight who is searching for his purpose in this strange land. And perhaps a group of musical mice that perform for her and tell her tales of Underland's history through their songs.
He might meet descendants of the Valyrian people. These could be people who have managed to survive the cataclysm that befell Valyria and have unique knowledge about the place and its history.
She could meet the strange and wonderful creatures of Underland. Maybe she has a tea party with the Mad Hatter and his friends, and they share their wildest stories. Or she could get involved in a quest to save Underland from a new threat that has emerged since Alice's last visit.
Goten could meet the younger versions of his friends like Krillin or Yamcha. They might be a bit skeptical of him at first. He could also run into Master Roshi, who would probably be interested in Goten's abilities and might try to teach him some old - school techniques. And there's a chance he meets Bulma, who could be intrigued by his futuristic knowledge and might even try to get some tech ideas from him.
Naruto would meet his younger self. This would be a really strange situation. He could also meet all the important shinobi from the past. For example, the First and Second Hokage. He might have a chance to learn from them directly, instead of just hearing about them in stories. Additionally, he could meet some of the characters who were only mentioned in passing in the original story but had a big influence in the past.
He could meet his future self. It would be an interesting encounter as he could learn from his future experiences and wisdom. Or he might meet people who have studied his past adventures and see himself through their eyes.
He might meet the future versions of Naruto and Sakura. Their relationship with him could be very different in the future, perhaps they are now leaders of the village and Sasuke has to adjust to their new status.
She could meet some of Klaus's old friends or enemies. For example, she might run into an old vampire acquaintance of Klaus who has some interesting stories to tell about their past adventures together.
During their travel to the past in the fanfiction, Harry's children may meet Hagrid in his younger days. Hagrid could show them around Hogwarts in a time when it was different. They might also meet some of the dark wizards that their parents fought against, like Voldemort in his early forms. This would put them in a dangerous yet exciting situation. Moreover, they could encounter some of the minor characters that are often overlooked in the main story but have their own interesting stories to tell in the past setting.
He might meet a powerful wizard who is the ruler of that dimension. This wizard could initially be hostile towards Natsu, seeing him as an intruder, but later they could become allies.
Korra could meet the founders of the different bending disciplines. She could meet the first water benders and learn about the purest form of water bending. This would not only enhance her own skills but also give her a deeper connection to her water - bending heritage.