In a darker take, even some of the younger students at U.A. High could die. This would be a very emotional and impactful event. It would show the harsh reality of the hero world and how not everyone is guaranteed to survive, no matter how much potential they have.
I think the students like Kaminari and Sero would be involved. Kaminari with his goofy personality might initiate some pranks, and Sero could use his tape quirk to set up some elaborate prank setups. Also, the teachers like Aizawa could be involved. He might play a prank on the students to teach them a lesson in a rather unorthodox way. Another character could be Todoroki. His rather stoic nature could be a great contrast to the chaos of a prank.
Definitely Todoroki. His ice and fire powers could be used in different ways against Godzilla. He could try to freeze parts of Godzilla to immobilize it or use his fire to counter any heat - based attacks Godzilla might have. Also, Recovery Girl could be involved in an interesting way. After any battles with Godzilla, she would be needed to heal the injured heroes, and she might also come up with some unique medical strategies based on Godzilla's physiology if she could study it.
Definitely Deku. His determination and continuous growth would make him stand out in college. He'd probably be at the top of his classes, always eager to learn more about quirks and heroics. Bakugo would also shine. His powerful quirk and competitive nature would lead to some intense rivalries and great character development in a college environment. And Todoroki, with his complex family background and dual - natured quirk, would have a lot of room for exploration in college as he tries to further master his powers and come to terms with his past.
All Might could be a likely candidate. He's the symbol of peace in the My Hero Academia world and has a strong sense of justice. If he were to get the Infinity Gauntlet, he would probably use it to protect the world and keep the balance between heroes and villains. His already incredible strength combined with the power of the Infinity Gauntlet would make him nearly unstoppable.
All Might from My Hero Academia could also be a good choice. His strength and experience would be valuable. And from Nanatsu no Taizai, Ban. Ban's immortality and combat skills could complement All Might's power. They might form an alliance because they both have a strong will to protect the weak.
Obviously, Izuku Midoriya will be the central character. His friends from U.A. High like Ochaco Uraraka and Tenya Iida are likely to be involved. They would be there to support or oppose Izuku depending on the nature of the spiral he's in. Also, All Might might play a role as Izuku's mentor, guiding him through whatever challenges the spiral presents.
Definitely Izuku Midoriya. He's the main protagonist in the original series, so he would probably play a crucial role in any 'Days of Future Past' type of story. He could be the one leading the charge to change the future.
Todoroki is another possibility. Given his complex backstory and his struggle with his two different powers, the mask could symbolize his journey to fully accept himself. It could be a catalyst for him to merge his fire and ice powers in a new and unexpected way, or it could hold the key to unlocking some hidden aspect of his family's past that impacts his hero identity.
Deku is often a popular character in 'My Hero Academia Flame On' fanfiction. His original quirklessness and then his development into a powerful hero make him a great candidate for stories involving new or enhanced flame - based quirks. Bakugo is also popular as his explosive quirk can be easily related to the idea of flames. Fans like to write about him further exploring the power of his explosions in relation to the 'Flame On' concept.
One of the most memorable characters could be the fan - made villain with a really unique quirk. Maybe they have the ability to manipulate time in a small area, which makes them extremely difficult to defeat. Their backstory of how they got their quirk and why they turned to villainy is also very engaging.