For a more unexpected choice, you could have the Arbiter from Halo as one of the main characters. He has a complex backstory and his own code of honor. In a crossover, he could team up with a Hunter Guardian. The Hunter's brute strength combined with the Arbiter's strategic thinking would make for an exciting story as they face challenges in both universes.
Definitely, Cortana could be a main character in a 'halo anime crossover fanfiction'. If she were in the world of Code Geass, she could help Lelouch with his strategic plans, using her vast knowledge and intelligence. Another option is the Elites from Halo. They could be the main characters in a crossover with My Hero Academia. Their physical strength and warrior culture could be contrasted with the superheroes in that anime, and they could have some interesting interactions with the students at UA High.
Master Chief could be an obvious choice for the human side. He's a well - known and powerful character. For the Covenant, the Arbiter would make a great main character. Their different perspectives and skills could create an interesting dynamic.
Obviously, Harry Potter could be one of the main characters. He could be the one to bridge the gap between the two worlds, using his courage and magic skills. Master Chief from Halo is also a great candidate. His combat experience and leadership would be valuable in the wizarding world full of new threats. Hermione would also be important as her intelligence could help in understanding the Halo technology.
The Spartans from Halo would also be great main characters in this crossover. Their super - human abilities and combat training would make them stand out in the Supreme Commander world. They could be the ones to teach the Supreme Commander forces about hand - to - hand combat and stealth operations, which are not as emphasized in the Supreme Commander gameplay.
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.
It could be a combined form of several of Spider - Man's classic villains. For instance, Doc Ock, Electro, and Sandman fused together by some dark magic. This new super - villain would have the intelligence of Doc Ock, the electrical powers of Electro, and the shape - shifting abilities of Sandman. Spider - Man would have to use all of his skills and perhaps some new ones he gains during the crossover to defeat this powerful adversary.
One idea could be that the Master Chief from Halo is transported to the Destiny universe and has to team up with a Guardian to stop a new threat that is using technology from both universes. For example, the Covenant and the Fallen might be working together under a powerful new leader.
In many fanfics like 'a new destiny fanfic', there could be a group of friends or comrades. They each have their own unique skills and personalities. For example, one could be a tech - savvy character who can hack into systems, another could be a strong fighter, and together they form a team that supports the main character in their journey.
It depends on the specific 'changing destiny fanfic'. It could be the main characters from the original work it's based on, like if it's a fanfic of a popular TV show, it might be the lead actor's character who is trying to change their destiny.
There might be a dragon named Blaze. He could be a strong and brave character, always leading the way in battles and adventures.