A new character could be introduced as the antagonist. This character could be a time - traveling bounty hunter who has been sent to capture Five while he is sick. The bounty hunter might believe that Five's sickness makes him a danger to the time - space continuum and wants to lock him away in some sort of time - prison.
The main antagonist could be a part of Five himself. His sickness could create an alter - ego within him that is more aggressive and dangerous. This alter - ego could start to take over Five's actions and make him do things that are harmful to his siblings and the world around him, while his siblings struggle to bring the real Five back.
Another plot could involve Five getting sick due to some side - effect of his powers overusing them in a short period. As he gets sicker, he starts to lose his memories of different timelines he has visited. The story could then focus on his journey to regain those memories with the help of his siblings.
I also like 'The Lost Years'. It fills in the gaps of the characters' lives that were not fully explored in the show. It's full of interesting backstories and character development.
One theme could be acceptance. Five could accept Vanya fully, powers and all, which would be a great comfort to her. He might tell her that her powers don't make her a monster but rather a unique and important part of the family.
The 'Umbrella Academy Five Returns Earlier' fanfiction might explore various aspects. It could focus on how Five's earlier return disrupts the existing timeline in the Umbrella Academy universe. Maybe it shows how the other characters react to his unexpected early arrival. For example, his siblings might be shocked and have to quickly adapt to his presence, which could lead to new alliances or conflicts within the group.
The 8th child in the Umbrella Academy fanfiction might be a character with a tragic backstory. They could have been separated from the others at a young age and grew up in a different environment. Their power could be something related to nature, like the ability to control plants. When they finally meet the Umbrella Academy, they bring with them a different perspective and a new set of skills that could change the dynamics of the whole group.
Kaos could be a great main antagonist. He might see Dark Spyro's power as a tool to finally achieve his evil goals and try to manipulate him.
Maybe a mysterious figure from outside the academy's timeline. This person is jealous of the power the academy holds and wants to use Ichigo's time - traveling ability to get inside and destroy it from within, believing that the academy's existence disrupts the balance of the universe.
One possible comfort scenario could be after a particularly difficult mission. Five, with his time - traveling experience, might share some wisdom with Vanya to soothe her. For example, he could tell her that the chaos they face is all part of a bigger plan and that she has an important role to play in the grand scheme of things. This would give Vanya a sense of purpose and comfort.
It could be a rogue ninja who has a vendetta against all the villages represented at the summit. He might have been wronged in the past by the combined actions of the five great villages and now seeks revenge. He could use the chaos of the summit to launch attacks and try to undermine the stability of the shinobi world.
It could be a rogue teacher at Duel Academy who has his own dark motives. He might be secretly researching dangerous dueling techniques and using students as guinea pigs for his experiments.