There could be some old and wise wizards or witches from outside of Hogwarts who come to help. Maybe they heard about Harry's heroics in the battle and feel obligated to use their ancient magic knowledge to save him. And, of course, Ginny would be by his side, using her love and determination to will Harry back to health.
Well, perhaps it could be about Harry being seriously injured during the Battle of Hogwarts. His friends might be scrambling to find a way to heal him. Maybe Hermione would be frantically searching through all her books for a healing spell while Ron stays by Harry's side, constantly worried.
His friend Ron could come back to help him. Ron is always loyal and might not want to leave Harry behind. He could bring his family's flying car again to pick up Harry.
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.
It could be a rogue Death Eater who managed to catch Harry off - guard. Maybe they were lying in wait and launched a surprise attack when Harry was alone, away from the safety of Hogwarts.
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.
Dumbledore could play a role too. He has vast knowledge and connections. He might give Harry some hints or direct him to certain places or people who can assist in finding the cure for lycanthropy.
Ron and Hermione are the most likely ones. They are his best friends and have always been by his side through thick and thin. They would probably follow him without hesitation and use their own skills, like Hermione's vast knowledge and Ron's loyalty and practical skills, to assist in the ship - stealing adventure.
I remember a fic where Harry and Draco end up on the couch post - battle. It was called 'Aftermath on the Couch'. The story delved into how their past rivalries still lingered but also how the shared experience of the battle made them realize they had more in common than they thought. It was really interesting to see how the author developed their relationship in that small, intimate setting of the couch.
Dumbledore might help in a spiritual or advisory way. Even though he's passed away, his wisdom could be passed on through memories or some magical means. Also, Hagrid could be a great help. He could assist in building areas suitable for magical creatures with his love and understanding of them.
His best friends Hermione and Ron would surely be there to help. Hermione with her vast knowledge of magic could research about the egg, and Ron with his loyalty would stand by Harry's side in any danger related to the egg.