Adding a mysterious traveler from a far - off land would be exciting. This character could bring with them unique weapons or skills that no one in the main game world has seen before. Also, a young and inexperienced hero - in - training who looks up to the main heroes could be added. Their journey of growing stronger and facing Calamity Ganon would be engaging.
You could include a powerful dragon that has been in hibernation for centuries. When awakened by the threat of Calamity Ganon, it decides to join the fight. Its massive power and unique abilities would surely make the fanfic more exciting. And perhaps a spirit of an ancient hero who can possess the main character temporarily to give them extra strength in the battle against Calamity Ganon.
The Borg Queen could be an interesting addition. Her hive - mindedness would contrast with Khan's individualistic and ambitious nature. There could be a battle of wills between them, with Khan trying to outwit the Borg Queen's plans for assimilation.
How about adding some characters from the Fate/Extra series? For instance, Nero. Her strong - willed personality and combat skills would surely spice things up when paired with Archer in the Fourth War. And then there's Tamamo - no - Mae. Her charm and tricky nature could lead to some really engaging storylines involving Archer.
One idea could be exploring Calamity Ganon's origin story in more detail. Maybe there was a tragic event in the past that led it to become the menacing force it is. Another could be a fanfic where the heroes have to face a different form of Calamity Ganon, one that has new powers and abilities. For example, a Calamity Ganon that can manipulate time.
A young detective who idolizes Batman from afar and tries to help him in secret. This character could bring a fresh perspective to the story and also add some mystery as Batman tries to figure out who this helpful stranger is.
One potential character could be a new sidekick for Batgirl Beyond. Maybe a young girl who idolizes Batgirl and has some unique skills like acrobatics or martial arts training. She could bring a fresh dynamic to the story and help Batgirl in her missions.
You could add a new demigod character. Maybe a child of Hecate who can use magic in interesting ways around the school. For example, she could make the lockers disappear or change the school cafeteria into a magical forest for a short time. Another character could be a satyr who is pretending to be a normal student but is really there to protect the demigod students. He could be the one who always seems to be eating strange things from his backpack, like enchanted herbs.
One potential character could be a wise old hermit who lives in a secluded part of the paradise. He could offer Percy valuable advice and knowledge about the secrets of the paradise.
A nomadic vampire with a completely different set of values compared to the Cullens. He could be more in tune with his animalistic side and not believe in the Cullens' way of trying to blend in with humans. His presence could cause conflict within the vampire community in Forks and also attract the attention of the werewolves.
One possibility is a young girl who has the ability to control light in a way different from Ruby's speed. She could be a street urchin who has been surviving on her own in the slums of a major city in Remnant. When she meets the RWBY team, she initially distrusts them but eventually becomes an important part of their group. Also, a grizzled old veteran of many battles against Grimm could be added. He could be a mentor figure, teaching the RWBY characters new combat techniques and sharing his wisdom about the history of Remnant and the Grimm.
You could add a young and inexperienced vault hunter who is eager to make a name for themselves. They might start off as a bit of a nuisance to the more experienced characters but gradually prove their worth. There could also be a mysterious merchant who is not what they seem. They might have a hidden agenda and be involved in some sort of illegal trading that could have a big impact on the Borderlands economy.