Potentially, a seer or a fortune - teller in the past could interact with Caroline. They might have some knowledge about her time - traveling situation or be able to give her clues on how to get back. Another character could be a young inventor or a magician's apprentice. They would be interested in Caroline's modern ideas and try to incorporate them into their work in the past. And her own ancestors, if she travels far enough back, would be very important characters. She might have to protect them from a threat in the past to ensure her own existence in the present.
One type of character would be the local townspeople in the time period she visits. They can show her the ropes of that era, introduce her to their culture and way of life. Then there are the antagonists. Maybe there's a power - hungry person in the past who wants to use Caroline's knowledge of the future for their own gain. Also, a love interest from the past. This person could be a nobleman or a simple farmer, but they would be drawn to Caroline's different nature and they would have a relationship that develops as she tries to navigate the past.
Jiraiya could also be a character that Karin interacts with. Jiraiya is always on the lookout for knowledge and new techniques. Karin, coming from the future, could have some valuable information for him. They might bond over their shared love for the Uzumaki clan's secrets or Karin could learn some new jutsu from Jiraiya in the past. And of course, Tsunade. Tsunade is a powerful figure in the Naruto world. Karin could seek her help or advice in the past, and Tsunade might be interested in Karin's future knowledge especially regarding medical ninjutsu which both of them are associated with.
Naruto, of course. Since they are in the same team in the original story, it's very likely that Sakura will interact with Naruto. She might try to change his fate or help him in a different way. Sasuke is also a main character she'll interact with. Maybe she'll try to prevent Sasuke from going down his dark path. And Kakashi, as their teacher, will also be involved in their interactions.
They are likely to interact with Naruto's past self. It would be an interesting dynamic as Naruto tries to explain things to his younger self without messing up the timeline too much. And Himawari would be there, perhaps teaching her young father some of her own jutsu.
Natsu would be a very likely character. Since he and Lucy are close friends in the original story, if Lucy travels back in time, Natsu might be one of the first people she seeks out. They could have adventures together in the past, with Natsu being his usual impulsive self and Lucy having to keep him in check with her knowledge of the future.
Shinji is a very likely character. His complex relationship with his father and his role as the main pilot make him a prime candidate for time travel scenarios. He could go back in time to try and fix things or understand his father better.
Tony Stark is a likely focus. His intelligence and experience with technology, especially time - travel technology in the movies, make him a central figure. He could be the one to figure out the mechanics of time travel and lead the Avengers in their time - traveling adventures.
Batman himself is a likely candidate. Given his intelligence and determination, he might stumble upon a time - traveling device or technology while investigating some strange phenomenon. He would be driven by his need to protect Gotham, perhaps by preventing a future catastrophe that he has foreseen.
Ban could be another one. He has his own backstory related to the sins and his immortality. He might time travel to find a way to break free from his immortality curse or to go back and correct a mistake he made when he was under the influence of one of the sins, like greed when he stole something important in the past.
Star - Lord is a likely candidate. He's always been a bit of a risk - taker and his curiosity might lead him to experiment with time travel technology.
I think Fundy would be likely to time travel. He has a complex relationship with his family and the SMP. Time travel could be a means for him to understand his father better or change the way his family relationships developed. GeorgeNotFound could also be a candidate. He's often in the background but has a strong connection to Dream. Time travel might be his way of getting more involved in the SMP's events and helping Dream in a different way. Lastly, Niki could be a time traveler. She has been through a lot of changes on the SMP, and time travel could be her opportunity to go back and make the SMP a more inclusive and friendly place from the start, especially for those who have been marginalized.