Luna Lovegood would be a great addition. Her unique view of the wizarding world and her knowledge of some of the more esoteric aspects of magic could be really useful during their time - travel journey. She might also offer some off - beat and humorous moments.
Definitely Albus Potter. Since he's Scorose's contemporary and is also related to the Potter family, he could add some interesting family dynamics to the story. His own experiences and abilities can play a role in the time - travel adventures.
Obviously, Sirius and Harry are the main ones. But including Remus Lupin can add depth. Since he was part of the Marauders and was close to Sirius, his presence can create some interesting dynamics. He can also offer his wisdom and werewolf - related knowledge to help in their time - traveling adventures.
Definitely Kushina should be included. She was an important part of Minato's life. Their relationship could be explored in new ways with the time travel element. For example, Minato could try to protect her even better from the start. Jiraiya is also a great choice. Minato could learn more from him earlier or even teach Jiraiya some of his future knowledge.
One idea could be that Scorose travels back to the time of the founders of Hogwarts. Rose could use her modern knowledge of magic to solve some ancient mysteries that the founders were facing, while Scorose's relationship develops in this new and different setting.
Definitely Marceline. Since she has a long - standing relationship with Simon (Ice King), her presence can add a lot of depth and emotion to the story. Their past connection gives a lot of room for interesting interactions.
Morgan could be an interesting addition. He might accidentally get caught up in Chuck's time - travel adventures. His comical nature could provide some light - hearted moments in an otherwise complex time - travel plot, like when he misunderstands the technology or customs of a different time.
One popular character type is the time - traveling scientist. They might be interested in studying the scientific knowledge or lack thereof during WW1. For example, they could try to introduce new medical techniques. Another is the adventure - seeking young adult. They see the time travel as an opportunity for a great adventure and end up getting involved in the war in unexpected ways. Also, a war journalist from a different era could be a great character. They could document the events of WW1 in a new light.
Issei himself is of course very interesting. His reactions to different time periods and how he uses his knowledge from the future can be really engaging. Then there's Rias. Her presence can add a layer of complexity, especially if she has different relationships with Issei in different timelines. Another interesting character could be Vali. His rivalry with Issei can take on new dimensions when time travel is involved, like competing in different historical battles or for the affections of a character in a different era.
Rick is an obvious choice. His intelligence and knowledge of the multiverse can add a lot of depth to the time travel story. He could be the one helping Morty fix the problems caused by time travel, or he could be the cause of the problems in the first place. Another interesting character is Summer. She could get involved in Morty's time travel adventures, and her different perspective as Morty's sister can bring new elements to the story. Maybe she's jealous of Morty's time - traveling and tries to tag along.
Gwen could be really interesting. She could have developed different powers in a different time period that Ben discovers when he travels back. Also, Grandpa Max in his younger days would be cool. He might have been involved in some secret time - traveling adventures that Ben uncovers. And Vilgax from different points in time could add a lot of drama. Maybe a younger Vilgax who is still learning and more vulnerable, or an older, more powerful Vilgax from a far - off future.
Littlefoot is the central character, but for some interesting twists, including some of the villains or less - friendly dinosaurs can be good. For instance, Sharptooth could be in the fanfiction not just as a mindless threat but with some kind of backstory that makes him more complex. Also, the old Threehorn leader who was a bit grumpy could add some conflict and humor to the story. And don't forget about the wise old dinosaurs in the Great Valley who can offer guidance to Littlefoot and his friends.