Naruto himself would probably be one of the main characters. Since he has always been about protecting his friends and the village, a 'greater good' narrative could put him in situations where he has to make even more difficult decisions regarding the well - being of everyone. He might have to sacrifice his own desires for the greater good.
Sasuke could also be a central figure. His complex character and his own sense of justice could be explored in relation to the greater good. He might start off with his own ideas of what's right, but as the story progresses, he could be influenced by the concept of the greater good and change his perspective.
Well, obviously Naruto himself would be the main character. There could also be his friends like Sasuke and Sakura, who might be involved in the interactions between the two Narutos. Maybe they are confused at first by seeing two Narutos, but then they help them figure out what's going on.
It could still be Naruto himself. Since he's the central character in the original series, many fanfics would likely continue to build on his story.
Well, Harry Potter himself would most likely be one of the main characters. He has always been at the center of the fight against evil in the original series, so in a fanfic about the greater good, he would play a crucial role. Hermione Granger could also be a main character. Her intelligence and loyalty would be important assets in any situation related to the greater good. Ron Weasley might be there too, providing his humor and courage. And perhaps some new characters could be introduced as well, like a young wizard from a far - off land who has unique magical abilities and a different perspective on what the greater good means.
Obviously, Naruto would be one of the main characters as the story is about him being the son of Whitebeard. Whitebeard himself would also be a major character, even if he might not be around for the whole story. And then there would be the other members of the Whitebeard pirates, like Marco. They would play important roles in Naruto's life in the pirate world, either as mentors or comrades.
Obviously, Naruto and Jiraiya are the main characters. They are the focus of the 'naruto meets jiraiya' concept. Naruto, the energetic and determined ninja, and Jiraiya, the wise and powerful mentor.
Naruto is surely going to be one of the main characters. After all, it's a Naruto fanfic. Sasuke is also very likely to be there as his rival and friend. Sakura might also play an important role.
Sasuke could also be a main character in the 'naruto jonin exams fanfic'. His skills and his rivalry with Naruto would make for an interesting plotline during the exams. Also, Sakura might be a main character as well. She has grown as a ninja and could show off her new abilities during the jonin exams.
From the Tsukune side, Tsukune would be there as the character being bashed. His friends from his own story might also make an appearance, but mainly to be overshadowed by the Naruto characters. For example, Moka could be in it, but the focus would be on how Naruto characters are better in various aspects compared to her and Tsukune.
Naruto himself could be a main character as the fanfic might explore how his life could have been different without the Sandaime's influence. Another likely main character could be Orochimaru, especially if the fanfic focuses on the Sandaime's failure to deal with him properly.
Obviously Naruto would be the central character as the title implies his actions towards dooming Konoha. Sasuke could also play a major role. He has his own complex relationship with Konoha and might side with Naruto, or be torn between stopping Naruto and understanding his reasons. Kakashi too could be a main character, as he has a deep connection with Naruto and would likely try to reason with him or oppose him if Naruto is set on dooming Konoha.