Naruto himself is very likely to have a significant role. He is the main character of the original series, and his strong will and unique abilities would be crucial in any time - traveling adventure. His determination to protect his friends and the ninja world would drive the plot.
Sasuke is very likely to have an important role. Since he and Naruto have a long - standing relationship, Sasuke's reaction to Naruto's Six Paths powers would be significant. He might be jealous at first but then come to accept and work with Naruto in a new way.
Green Lantern could be another significant character. He could introduce the Batman sons to the idea of using willpower and the power of the ring. Aquaman might be involved too, perhaps showing the sons the importance of protecting the oceans. And Martian Manhunter, with his vast powers and wisdom, could offer unique insights to the sons and help them understand their place in a bigger universe.
Naruto is a very likely character. Given his strong will and connection to the Nine - Tails, he would be motivated to time - travel to fix problems related to the Juubi. His ability to make friends and influence people would also play a big role in any time - traveling story.
Sasuke is a very likely character. He already has the Sharingan and has a strong connection to the Uchiha heritage. In fanfiction, it's easy to imagine him unlocking the Mangekyou Sharingan's time - travel ability through some intense emotional experience or a new discovery about his clan's powers. Another one could be Itachi. Given his complex backstory and his powerful use of the Sharingan, it would be interesting to see him use a time - traveling Mangekyou Sharingan to change things like the decisions he made regarding the Uchiha clan.
Sasuke would also be a key character. His past actions during the war need to be reconciled, and he could play a role in training the next generation of ninjas, passing on his unique skills.
Obviously Naruto himself is the central character. His growth and new abilities would be the focus. Sasuke could also be important. He might have his own development and his relationship with Naruto could be explored further, whether they are friends or rivals again.
Naruto himself could be a likely time - traveler. He has a strong sense of justice and a desire to protect his friends and the village. If he saw a way to go back in time and prevent the destruction and loss in the Fourth Shinobi War, he would probably take it. Plus, his chakra and determination might be what enables the time - travel in the first place.
I think Yamato could be a good choice too. His ability to control wood style could be really useful in the past. Maybe he could time - travel to help with the growth of the first trees in the ninja world or something like that. And his calm and collected nature would help him adapt to the new situation in the past.
Well, Naruto would be the one to bring hope and his unique brand of optimism to the past. He could show the younger versions of characters how to be a better ninja. Sasuke, being the brooding and intelligent one, would analyze the situation of the past and figure out how to use their time - traveling advantage to the fullest. Sakura, she'd be a key player in both the medical and combat aspects. She could teach the past medics new techniques. And Kakashi, he's the experienced one. He'd be making sure the team doesn't make any rash decisions that could mess up the timeline further.
Definitely Kakashi. He's already very intelligent and has a wide range of skills. Time control could be an addition to his arsenal, and he could use it in creative ways in battle or for teaching purposes. Also, Madara Uchiha could be a likely character. He was always seeking more power, and time control would be a very powerful ability for him to have in fanfiction.