Jiraiya would be an interesting character. His relationship with Naruto could be explored in a new light if Naruto travels back in time. Jiraiya could teach Naruto different things or Naruto could use his future knowledge to help Jiraiya in his missions.
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.
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 of the most interesting characters to include would be Orochimaru. His thirst for knowledge and immortality could lead to some really wild plot twists in a future time - travel fanfiction. Then there's Shikamaru. His intelligence could be used to strategize in a future full of new challenges. And Sakura. Her growth as a ninja and her relationship with Naruto and the others could bring a lot of depth to the story.
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.
Definitely Naruto. Even if it's a Team Konohamaru time travel fanfiction, Naruto's presence can add a lot of excitement. He could offer his unique perspective and skills, and his interactions with Konohamaru and the team would be really interesting, especially if they travel to a time when Naruto was younger or not even born yet.
Percy himself is always an intriguing character to include. His sarcastic humor and his ability to get into trouble and out of it are fun to write about. Annabeth is also great. Her intelligence and her strategic mind can add a lot to the story. Grover is another one. His loyalty and his goat - like tendencies can create some humorous and heartwarming moments. And don't forget about the villains. A well - written villain like a powerful monster or a devious demigod who has a vendetta against Percy can make the story really exciting.
Well, in a Naruto time travel future fanfiction, a plotline could be Naruto finding himself in a future where the chakra system has been completely disrupted. People are suffering because of this. He uses his time - traveling knowledge to find a way to fix it. Another idea is that he travels to a future where the five great nations are at war again, but this time it's a more technological and brutal war. He has to use his experience and charisma to stop the war and bring peace once more.
One interesting plot could be that when they time travel, they end up in a time before the Uchiha massacre. Itachi could then try to change things with Naruto's help, and in the process, their relationship deepens in a yaoi way. Maybe Naruto helps Itachi see a different path for the Uchiha, and they grow closer as they face the challenges together.
In some fanfictions, Naruto time - travels and ends up being adopted by the Uchiha family. This gives him a whole new perspective on the Uchiha's situation. He then uses his position to try and prevent the misunderstandings that lead to their destruction. There are also fanfictions where Naruto travels back to meet Madara Uchiha as a young man. He tries to change Madara's view of the world and stop him from starting the cycle of hatred that plagues the ninja world.
The outcast warlord is always an interesting character. Shunned by the other warlords, he has to rely on his own wits and the loyalty of a few followers. He could be hiding some amazing abilities or a great secret. Then there's the warlord who is a master of disguise. He can infiltrate enemy camps easily and gather crucial information. This makes him a key player in the power struggles. Also, the warlord with a split personality. One side of him is kind and just, while the other is ruthless and power - hungry. This internal conflict adds depth to the character and the story.