I think Nami would be a great character to focus on. Given her love for money and maps, if she travels to the past, she could discover hidden treasures that no one has found in the present. In the future, she could learn about new navigation techniques and bring that knowledge back. Also, her more cautious and calculating side would contrast with the chaos of time travel. She would be the one trying to figure out how to get back home while also making sure the crew doesn't get into too much trouble.
Luffy would be really interesting. His carefree and adventurous nature would make his reactions to different time periods really fun to explore. He might try to befriend everyone he meets in the past or future, regardless of the consequences. His simple view of things could also lead to some unexpected changes in the timeline. For example, he might accidentally start a new pirate crew in the past or inspire a revolution in the future just by being himself.
One idea could be that the Straw Hat Crew travels back in time to the era of the Pirate King Gol D. Roger. They could meet him and learn some of his secrets, or even witness some of his greatest battles. Another plot could involve traveling to a time when the world was in a different state, like before the Grand Line was discovered, and they have to adapt to a whole new set of challenges and rules. Maybe they accidentally change something in the past that has a huge impact on the present and they have to figure out how to fix it.
Mr. Incredible would be really interesting. He's already a strong and experienced hero. If he travels through time, he could use his brute strength in different historical contexts. He might have to adapt his fighting style in eras with different technologies or combat techniques. For example, in a medieval - like time period, he'd have to rely more on his physical skills without his modern gadgets. Also, seeing how he reacts to meeting his ancestors or versions of himself from different times would add depth to the story.
Reaper could also be an interesting focus. His dark and mysterious past could be explored further through time travel. He could go back to the time when he was still Gabriel Reyes, before the transformation. This could show the events that led to his current state, and perhaps in a time travel scenario, he could even have a chance to change his fate. It would be a story full of drama, mystery and action as he grapples with his past self and the decisions that led him to become Reaper.
I think Kai would be a great focus. His hot - headed nature could lead to some really chaotic time - travel situations. For instance, if he travels back to a time when the Fire Temple was in danger, he might rush into action without fully understanding the consequences, and end up changing the future of the Fire Element in Ninjago in unexpected ways.
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.
Captain America would be really interesting. His moral compass and leadership skills in a time - travel scenario would be fascinating. He has to deal with the changes in the past that the time travel causes, and also make sure the other Avengers stay on the right path. He might struggle with seeing his old world again and having to make choices that could change it further.
Kuroko himself would be very interesting. His quiet and somewhat mysterious nature could be explored in new ways with time travel. He could go back to a time before he met the Generation of Miracles and use his new - found knowledge to help his younger self become an even better player sooner.
Pepper Potts could also be very interesting. If she travels back in time, she could use her business acumen and knowledge of the future Stark Industries to make significant changes. She might also have a different relationship with Howard Stark, influencing the Stark family in new ways.
Natsu could be really interesting. Imagine him traveling back in time and accidentally causing chaos because of his impulsive nature. He might try to use his fire magic in a time when it was less understood or accepted, and have to deal with the consequences. He could also meet younger versions of his friends and have heart - warming or hilarious interactions.