Dumbledore could potentially be a source of help. He is extremely powerful and wise. He could have given Harry some advice or a special magical item before the encounter with the dragon that would prove crucial in Harry's ability to kill the dragon.
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.
Yes, there are likely fanfics out there with that plot. Fanfic writers often explore unique and creative scenarios like Harry killing a dragon. You can try searching on fanfic platforms such as Archive of Our Own or Fanfiction.net to find such stories.
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.
Hermione would most likely help. She's always been there for Harry, and with her vast knowledge from books, she could offer a great deal of advice on how to take care of the dragon egg. Ron might also be involved. Despite his initial fear of things, he has a brave heart and would probably stand by Harry's side.
One consequence could be that he becomes a hero among his peers. Everyone in Hogwarts might look up to him even more. The other students could see him as a real dragon - slayer, which gives him more popularity and respect.
Well, perhaps in this fanfic, Harry has some special equipment that helps him kill the dragon. He could have found a magical sword that is especially effective against dragons, and with his courage and quick reflexes, he manages to slay the dragon. It might also be that he has the help of his friends in some way, like Hermione providing some crucial knowledge about the dragon's weakness.
Well, perhaps the dragon is terrorizing a village near Hogwarts. Harry, being the brave hero, decides to take it upon himself to stop the dragon. He uses his magic skills, like the Patronus charm to disorient the dragon, and then a powerful blasting spell to finish it off.
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.
It could be Norbert, the dragon Hagrid once had. Since Norbert was a young dragon when we first saw it in the original story, it might have grown up and have a lot of stories to tell Harry.