Well, maybe Harry uses his magic skills like the Patronus charm to fend off some dark force attacking Xavier's Mansion. He could team up with the mutants there, and they'd be amazed by his ability to apparate.
Perhaps in this fanfiction, Voldemort has somehow made an alliance with a powerful mutant enemy of Xavier's. Harry, being the hero he is, travels to the mutant world. He first has to figure out how to use his magic in a world full of strange powers. He then uses spells like Expelliarmus to disarm the villains threatening Xavier's Mansion and saves the day with the help of the X - Men who learn a lot about a different kind of power - magic.
Maybe Magneto could be the villain in a sense. He could have a misunderstanding about Harry and think he's a threat to mutants. So he attacks Xavier's Mansion while Harry is there, and Harry has to convince Magneto that he's there to help and not harm mutants. And there could be a more powerful dark force manipulating Magneto from behind the scenes.
Well, in this fanfiction, Harry could be trained in a very different way. Xavier is known for his wisdom and his ability to guide young mutants. He might introduce Harry to concepts like acceptance and co - existence at an early stage. Since Xavier has telepathic powers, he could potentially shield Harry from some of the negative influences that Voldemort might try to impose on him psychologically. And Harry might learn combat and self - defense skills in a way that combines magic and mutant - like abilities.
The plot could start with Harry somehow ending up in Gotham. He notices the Joker's chaos and decides to intervene. Harry might use his invisibility cloak to get close to the Joker without being detected at first. Then he could start using his magic to slowly disrupt the Joker's evil plans.
One possible plot could be that a powerful dark wizard has set his sights on the Cullens for some reason, maybe to use their vampire powers for his own evil deeds. Harry, being the hero he is, senses this danger through his connection to the magical world. He then teams up with the Cullens, teaching them some basic wizarding defenses like Shield Charms. He also uses his own powerful magic, such as the Killing Curse in a defensive way if necessary, to save the Cullens from the clutches of this dark wizard.
Maybe Harry uses his magic skills like the Patronus charm to protect the Jedi from the Sith. His ability to apparate could also help Jedi in quick escapes.
Well, perhaps the vampire is being hunted by some dark wizards. Harry Potter, being the hero he is, uses his magic skills like the Patronus charm to fend off the attackers and save the vampire. Maybe the vampire has some crucial information about Voldemort that Harry needs.
In a 'Harry Potter Saves Bellatrix' fanfiction, perhaps Bellatrix is in a situation where she's about to be killed by other Death Eaters who think she's a liability. Harry, having a sense of justice and seeing the potential for redemption in her, steps in to save her. He could then try to show her the error of her ways, using his own experiences of losing loved ones to Voldemort's ideology. Bellatrix might initially resist, but gradually come to understand Harry's point of view.
The plot could start with Harry Potter noticing strange magical anomalies. These anomalies turn out to be related to Maximoff's world. Harry decides to investigate and finds Maximoff in a dangerous situation. He then uses his magical abilities to rescue him.
Well, perhaps Harry stumbles upon a goblin in a dangerous situation, like being trapped by dark wizards who want to steal the goblin's magic artifacts. Harry uses his skills and spells to fend off the bad guys and rescue the goblin.
One possible plot could be that the Queen is in danger from dark wizards who want to take over the Muggle government. Harry, using his skills from Hogwarts, discovers the plot and uses magic to protect the Queen. He might use spells like Protego Maxima around Buckingham Palace.