A plot could start with Theoden being poisoned. Harry Potter, who has a natural inclination for helping those in need, arrives in Middle - earth. He tries to figure out the nature of the poison using his magical knowledge. He might use a spell like 'Revelio' to discover the components of the poison. Then, he uses a combination of magic and the local herbs and remedies to create a cure. As he heals Theoden, he also gains the respect and friendship of the Rohirrim, and they in turn help him find a way back to his own world.
Well, perhaps Harry gets lost in a magical portal and ends up in Middle - earth near Rohan. He finds Theoden ill or injured. Harry uses his magic, like the spell 'Ferula' to bind wounds or 'Brackium Emendo' to mend broken bones. Then he has to deal with the Rohirrim's suspicion of his strange magic.
There are quite a few. A lot of these fanfictions explore the idea of how different magical systems interact. In the case of Harry Potter healing Theoden, it might involve Harry having to adapt his magic to the more medieval - like setting of Middle - earth. Some fanfics focus on the emotional and cultural exchange that comes with such an event, not just the physical healing.
In such a fanfiction, perhaps Harry uses his magic skills learned at Hogwarts. He might first find the injured unicorn in the Forbidden Forest. Then, using a spell like 'Episkey' or some advanced healing magic he's discovered. The unicorn could be injured by dark creatures or a trap set by poachers.
The fanfiction might start with Harry being haunted by the fate of the Longbottoms. He realizes that he has a moral obligation to try and help them. As he begins his journey to heal them, he uncovers secrets about the dark magic that was used against the Longbottoms. These secrets could be related to a particular Death Eater or a forgotten branch of dark magic. Along the way, he has to also deal with his own insecurities and doubts, but with the support of his friends, he perseveres until he finally manages to heal the Longbottoms.
One possibility is through magic. Harry Potter is known for his powerful magic skills. He could use spells like 'Episkey' or more advanced healing magic. Maybe he stumbles upon Thranduil who is injured in some enchanted forest. Harry, being the kind - hearted hero, decides to use his magic to heal the elven king.
One way it could happen is through magic. Harry Potter is well-versed in powerful magic. He could use spells like 'Episkey' or more complex healing magic to mend Pietro's injuries. Maybe he'd have to study some new spells specific to Pietro's unique speedster physiology.
One way it could happen is through the power of the Time - Turner. Harry could use it to go back in time and prevent the event that injured Tony Stark in the first place. Or he could take Tony to Hogwarts where the advanced magic and potions could be used to heal him. The magic potions like the Felix Felicis could be given to Tony to enhance his chances of a speedy recovery, and the healing spells of Madam Pomfrey could also be very effective.
One reason could be for the greater good of the wizarding world. If Lucius is healed, he might be able to turn away from the Dark side and provide useful information about Voldemort's remaining followers.
Dumbledore might also play a role. He has vast knowledge and experience. He could offer some wise advice or even provide a special magical item or spell that is crucial for the unicorn's healing.
The plot could involve a situation where a new form of dark magic is unleashed in the wizarding world, which spreads like wildfire and causes destruction similar to 911. The Ministry of Magic is caught off - guard. Hermione, with her vast knowledge, starts researching a way to counter this magic. Meanwhile, Ron and Harry lead the effort to protect the civilians. They set up shelters in places like the Room of Requirement and use Patronuses to guide people to safety. As they dig deeper, they discover that this dark magic was a planned attack by a long - hidden dark wizard group who wants to take over the wizarding world.
In some fanfictions, Snape might use his advanced knowledge of potions to create a special concoction to heal Harry. He could draw on his skills in Herbology as well to find rare ingredients for the potion.