Ruby would be really interesting. Since she's the main character, seeing her go back in time and interact with her younger self or her mother could be really emotional and full of character development. She could also use her knowledge to make herself an even better Huntress at an earlier age.
Weiss could also be a great choice. If she travels back in time, she could change the way her family has been treating the Faunus in the past, which would have a huge impact on the social and political situation in Remnant. It would also be interesting to see her deal with her past self's insecurities and prejudices.
Kazama Chikage might be an interesting choice as well. He is a powerful and somewhat arrogant character. If he time - travels, perhaps to a past era where his clan was even more powerful, he could try to change history to make his clan even more dominant. Or if he travels to a future where his power is nothing compared to modern technology, his struggle to come to terms with his new position in the world would be an interesting plotline.
Iroh would be fascinating to see time - traveling. He has so much wisdom already. If he traveled to different times, he could share his knowledge and perhaps change some key events. He could also gain more insights from other time periods, which would further enrich his already deep understanding of the world.
Daenerys Targaryen. She has a strong will and a mission to claim the Iron Throne. If she time - traveled, she could meet her ancestors and learn from their successes and failures. She could also try to prevent the downfall of her family in the past, which would have a huge impact on the present state of Westeros in the fanfiction world.
I think Izuku Midoriya would be really interesting. He's the main character and has so much knowledge about the heroes and villains. If he time - travels, he could use his understanding of quirks and combat strategies to change the past or future in significant ways. For example, he could go back and give advice to All Might when he was younger, which might change how All Might deals with villains later on.
Ahsoka Tano would be a great choice. She has a complex history within the Clone Wars. If she were to travel back in time, she could interact with her younger self. She might be able to pass on wisdom she gained from her later experiences, like how to better handle the politics within the Jedi Order and the Galactic Republic.
I think Zoro would be really interesting to time travel. He has a strong sense of honor and his swordsmanship skills are top - notch. If he traveled to the past, he could learn from the greatest swordsmen of that era and bring back ancient sword techniques to the present. In the future, he could see how swordsmanship has evolved and maybe even find a way to further enhance his own skills.
Hermione would be really interesting. She's so intelligent and studious. If she traveled back in time, she could use her knowledge of magic history and spells to make a big impact. For example, she might be able to help the Order of the Phoenix in new ways during the First Wizarding War or she could introduce some modern magical concepts to the wizards of the past.
Oscar could be an interesting main character for a time travel RWBY fanfic. As Ozpin's current host, he has a connection to a long history of events in Remnant. Time traveling could allow him to learn more about Ozpin's past directly, and he could also use his new - found knowledge to change things for the better in the present and future of Remnant.
Eleven could be a likely candidate. Given her powerful psychic abilities, she might accidentally or purposefully time travel. Her powers have always been a bit unpredictable, and time travel could be an extension of that. She has a strong connection to the Upside Down, so she might be sent back in time to learn more about its origin or how to stop it.
Shikamaru might be the most influential. He is known for his intelligence. In a time travel situation, he could quickly analyze the situation, come up with plans, and predict the consequences of different actions. His strategic mind would be crucial in dealing with any unexpected events during the time travel, whether it's in the past or the future.