A young scientist would be a great main character. They could be curious about what the future holds for their research and accidentally stumble upon a way to travel to the future. Their scientific knowledge would help them understand the new technologies and concepts in the future world.
A journalist could also be a main character. They are sent to cover a story about a new discovery related to time travel and end up in the future. Their job is to report back on what they see, which makes for an interesting perspective as they try to document this strange new world.
Obviously, the Marauders themselves - James Potter, Sirius Black, Remus Lupin and Peter Pettigrew would be the main characters. They are the ones 'meeting' the future after all.
Definitely Captain America. Their relationship has always been complex and full of different ideologies. Meeting future Tony would be a big moment for Steve as he would see how Tony has changed or what new ideas he brings. Also, Thor would likely interact with him. Thor, with his Asgardian knowledge and power, would be interested in the future technology Tony might bring. And of course, Black Widow. She's the master of espionage and would be curious about Tony's motives and what he's hiding from his future.
It could be a future version of an existing villain who has somehow gained even more power. For example, a more advanced and menacing version of the Light. They might be the ones causing the future problems that Future Robin is trying to prevent, and their actions in the present are just the start of their grand plan.
Naruto's children would likely be main characters. They would inherit his strength and determination. Also, the students of the new generation of ninja academies could play important roles as they strive to make their mark in a world shaped by Naruto and his peers.
The main character could be an android. In this future, androids have gained sentience but are still treated as second - class citizens. This android starts a revolution to gain equal rights for all artificial beings.
They are likely to meet their future selves. This could be a very strange and enlightening encounter for both of them. They might also meet their descendants, like their children and grandchildren, which would be a heartwarming experience.
He could meet his future self. This encounter might be full of surprises as his future self could have different views and experiences. His future self might be more jaded or more powerful.
Perhaps he would meet historians or scholars of his own time. They could have traveled to the future too and are now studying how his actions in the past have led to the present - future situation. It would be an interesting encounter as they could discuss the different interpretations of his life over time.
He could meet his future self. His future self might be wiser, having learned from all the experiences in the past. They could have a deep conversation about Shinji's purpose and the mistakes he's made.
She could meet her future self. This would be an interesting encounter as they could compare their experiences and knowledge. Her future self might give her some advice on things she should do or avoid in her own time. Another person she could meet is a descendant of her friends. For example, a grandchild of Harry and Ginny who has inherited their bravery and magical skills.