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.
Some of the Hogwarts professors might assist Harry. Professor McGonagall, for instance, has always been protective of Harry. She could use her position at Hogwarts to cover for Harry's activities when he is away from the school on his revenge - seeking missions. And Neville, who has grown into a strong and brave wizard, could also be part of Harry's team, using his knowledge of Hogwarts' secrets to their advantage.
One possible plot could be that Harry discovers some deep - seated corruption within the Ministry. He starts by gathering evidence secretly. Then, he uses his skills and the help of his friends to expose the wrongdoings in a very public way, perhaps through a magically - enhanced news broadcast that reaches every wizarding household, causing great embarrassment to the Ministry officials involved.
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.
His friends from Hogwarts like Hermione and Ron would most likely support him. They have always been by his side through thick and thin in the original story, so in the fanfiction, they would continue to support Harry in his fight against the Ministry.
The Order of the Phoenix members might also back Harry. They've fought against the dark forces before and if they see that the Ministry has become a threat, they'd side with Harry. Dumbledore's old allies in the Order would believe in Harry's judgment and join him in his mission to destroy the corrupt Ministry.
It could be the Ministry of Magic. They have often been shown to be corrupt and self - serving in the original series. If they try to manipulate Harry again, he might see them as the main antagonists and target them in his revenge.
Sometimes, new original characters are created in fanfiction to help Harry. These could be students from other wizarding schools who have unique skills. For example, a student from a school in a far - off land who has mastered a type of magic that is unknown in Hogwarts and uses it to help Harry in his fight against the dark forces.
Neville could be another helper. He has become quite skilled in herbology. He could help Harry by creating a magical garden around the house. The plants in the garden could be used for protection, like thorny bushes that keep unwanted visitors away, or for providing ingredients for household potions.
Dumbledore might help from beyond the grave, in a sense. Harry could discover some of Dumbledore's old notes or memories that provide crucial insights into building a powerful and controlled bomb. And perhaps some of the more eccentric professors at Hogwarts, like Professor Flitwick, could offer their expertise in charms which might be needed for the bomb's construction.
Harry Potter himself would likely be a central figure. He has a history of fighting against dark forces and would probably step up to try and maintain order. Hermione would also be important as she is intelligent and could come up with plans to deal with the situation. Ron, with his loyalty and courage, would be by their side.
It could be Dumbledore. He's always been like a mentor to Harry, so it would make sense for him to be the one to give an important talk, whether it's about Harry's destiny or how to handle the challenges in the wizarding world.