It could be a minor character from the original story. For instance, a servant who has always been in the background but suddenly gets thrust into the center of events and has to change their own destiny as well as those around them.
A new character introduced specifically for the fanfiction could also be the main protagonist. This character could come from a different realm or have a unique ability that allows them to interact with the existing world and its destinies in a new way. They might be an outcast in their own place and see the 'a change of destiny' world as a chance for a fresh start.
A lost princess could be the main character. She was supposed to marry a cruel prince as part of her pre - arranged destiny, but she rebels. She disguises herself, travels the land, and meets different people who help her change her destiny from being a trapped royal to a strong and independent leader of her people.
The Traveler itself could be considered the main character in a sense. The fanfiction could be told from its perspective as it awakens and experiences the world anew. It could show its thoughts, its power struggles, and its hopes for the Destiny universe as it tries to reshape it.
The main characters might include a group of outcasts. For example, a droid with a unique personality that has been rejected by its creators, a smuggler with a heart of gold, and a Jedi who has been exiled for her unorthodox beliefs. Together, they could be on a quest to change the destiny of the Star Wars galaxy as we know it. There could also be a Sith - turned - good character who is trying to make amends for his past deeds.
A young alien with a special gift for sensing the changes could be a main character. This alien could be from a species that has been living in harmony with the universe for a long time but now has to face the new, unexpected changes. Another option could be a rogue adventurer who has been traveling through different parts of the universe and has seen a lot of the changes first - hand. They could be a bit of a loner but end up having to team up with others to deal with the overarching issues of the changing universe.
Obviously, the Goblin Slayer himself would be one of the main characters. From Destiny, the Guardian, especially a Hunter with their stealth and agility, could be a great match. The Hunter's ability to quickly dispatch enemies and traverse difficult terrain could complement the Goblin Slayer's more brute - force approach when it comes to goblin - slaying.
A powerful ancient ghost could be the main antagonist. This ghost might have been locked away for centuries but has now been released and sees Danny's 'haunted destiny' as a threat to his own plans for the ghost zone. He could try to manipulate Danny's destiny to his advantage.
It could be another sister. Sometimes in stories, sibling rivalry can be a strong source of conflict. The other sister might be jealous of the main sister's growth and change for the better and try to undermine her at every step.
Kagome might be the main character. As she's from the modern world and has a big influence on the events in the feudal era, she could be the one who gets a chance to change the fates of everyone. For example, she could use her modern knowledge to change the course of battles or relationships in the story.
Eli Wallace would likely be one of the main characters. He's smart and resourceful and has been important in previous Stargate Universe stories. Also, Colonel Everett Young, as the leader of the group, would probably play a major role in this 'Back to Destiny' story.
Shikamaru is also a likely candidate. His high intelligence could be the key factor in this 'different destiny' story. He might use his strategic mind to outwit enemies that were previously thought to be unbeatable. For instance, he could come up with a plan to defeat Madara without relying on brute force, which would make him the central figure in the fanfic.