Obviously, Harry Potter himself is very likely to be the focus. Since it's a Harry Potter fanfic and the 'here we go again' concept can be related to his experiences. He might be the one reliving parts of his life or facing new challenges repeatedly.
Another likely character is Albus Dumbledore. He is a very mysterious and powerful character. In this type of fanfic, he could be the one behind the repeating events, either as a test for Harry or for some greater purpose that is slowly revealed as the story progresses.
Hermione Granger could also be a central character. Her intelligence and problem - solving skills would be crucial in trying to figure out why they are in a 'here we go again' situation. She could be the one devising plans to break the cycle or make the best of the repeated events.
One common theme could be time - loops or repeating events, just like the 'here we go again' implies. Maybe Harry has to relive a certain part of his Hogwarts years over and over until he gets it right. Another theme might be self - discovery during these repeated scenarios. He could be learning new things about himself or his friends each time the cycle restarts.
Harry Potter himself is a likely candidate. He has endured so much throughout his life, from the death of his parents to the constant battles against Voldemort. Fanfiction could explore his post - war trauma and how he tries to lead a normal life.
Obviously, Harry Potter himself is a likely hero. His experience in fighting Voldemort has given him the skills and courage to face new apocalyptic threats. Hermione Granger is also a strong candidate. Her intelligence and vast knowledge of magic would be invaluable in trying to solve the problems causing the apocalypse. Ron Weasley, with his loyalty and ability to think on his feet, would be right there beside them, fighting against whatever doom is approaching.
I think Luna Lovegood could also be included. Her unique perspective and knowledge of the magical world would add an interesting element to the road trip. She might lead them to discover some really strange and wonderful things.
One idea could be to explore a different timeline within the 'here we go again' universe. Maybe a timeline where the main characters made different choices at a crucial moment. For example, if in the original story they chose to fight, in the fanfic they could choose to negotiate instead, leading to a whole new set of adventures and relationships. Another idea is to focus on a side character from the original and give them a more central role in the fanfic. This could bring out new aspects of the story that were not explored before.
One common theme could be adventure. Since it has 'again' in it, perhaps the characters are going on another adventure through the gate. Another theme might be self - discovery as the characters face new challenges in this repeated scenario.
Harry Potter himself could be a likely focus. His status as the Boy Who Lived and his powerful magical abilities would make for an interesting reincarnation story. He could be reincarnated into the Bleach universe and become a powerful Soul Reaper or something unique within that world. Maybe he'd use his Gryffindor courage in new and different ways.
Definitely Captain America. He's always been a leader and in the aftermath of Infinity War, his leadership would be crucial in keeping the remaining heroes together. Also, Thor, who has lost so much, would be a great focus. His journey of dealing with his losses and finding a new purpose could be really interesting.
Obviously, Harry Potter and Clark Kent would interact a lot. They are both iconic characters in their own universes. Clark, with his superpowers, would be curious about Harry's magic, and Harry would be interested in Clark's abilities. Also, Hermione Granger, if she somehow also ends up in Smallville, would likely interact with Chloe Sullivan. They are both intelligent and inquisitive, so they could bond over solving mysteries or studying the differences between their two worlds.
Ron and Uryu might be paired. Ron has his own insecurities but also his moments of great courage, much like Uryu. They could bond over their skills in combat, Ron with his magic and Uryu with his Quincy powers.