Maybe it's an organization that is trying to take over the ninja world. They have already caused a lot of chaos and pain, and Boruto decides to take them on to avenge all those who have been affected by their actions. For example, they might have kidnapped and killed some of Boruto's friends or mentors, which enrages Boruto.
There's a possibility that the target is a powerful and corrupt shinobi from another village. This shinobi has been undermining Boruto's village's peace and prosperity. Boruto, seeing the damage done to his home and people, sets out to get revenge. He may start by challenging this shinobi in various ninja battles and gradually wear him down until he can finally defeat him.
It could be a rogue ninja who has a personal vendetta against Boruto's family. This ninja might have caused harm to Boruto's father Naruto in the past, and now Boruto is determined to make him pay.
It could be a powerful and evil ninja from a rival village who has killed one of Boruto's close friends or family members.
Perhaps the target is Howard. Howard might have made a joke about Sheldon's mother that Sheldon didn't appreciate. As a result, Sheldon could try to get revenge by interfering with Howard's work on the space station projects, causing minor but annoying setbacks.
Well, maybe Boruto's family or friends are hurt by a powerful enemy. So he decides to train hard and seek revenge. He might travel to different places to learn new jutsu and gather allies along the way.
In addition to his family and friends, the medical ninja in the village would be involved. They would be the ones to accurately diagnose Boruto's condition. Shizune, for example, could come and check on him, using her vast medical knowledge. And the other young medical ninjas might learn from this case, as Boruto's illness could be something new or unusual, so they would be eager to assist and gain experience while taking care of him.
In a 'Boruto Seeks Revenge Fanfic', Boruto might seek revenge against an enemy who has wronged his family. For example, if a new villain attacks Konoha and harms his father Naruto. Boruto could train intensively to gain power and then set out on a journey to find and defeat this foe.
It could be the Kara organization. They've been shown to be interested in Boruto's Karma mark in the original series, so they might have finally made a move to abduct him in the fanfic.
Creatures that have been wronged by Dumbledore's decisions in the past could side with Rumple. For instance, if Dumbledore once restricted the rights of house - elves, the house - elves might assist Rumple in his revenge plan, believing that Dumbledore needs to be taught a lesson.
Dumbledore, even if not in person, could have left behind some clues or resources that help Harry. Also, the Order of the Phoenix members who are still around and believe in Harry's cause. They have their own skills and connections that could prove useful. For example, Moody could teach Harry some advanced defensive magic techniques to use against the Ministry's security measures.
In the 'Boruto Gets Sick' fanfiction, Boruto might experience various symptoms like a high fever, weakness, and coughing. His friends and family would likely be worried about him. They would probably take care of him, bring him medicine and try to make him comfortable.
Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.