Perhaps some new dark wizards or witches who have a vendetta against Harry. They could be more powerful or have some unique tracking abilities that make it difficult for Harry to stay hidden anywhere else but the power station. They might be after him because they think he has some sort of valuable magical item or knowledge.
It could be Death Eaters. They've always been after Harry in the original series, so in this fanfiction, they might be hot on his trail again, forcing him to seek refuge in the power station.
Maybe Harry is on the run from some dark forces and he finds the power station as an unexpected hiding place. He could be using the complex structure and machinery of the power station to evade detection, perhaps with some magical twists like using spells to blend in with the electrical currents or machinery.
It could be Draco Malfoy. In an alternate universe, they could have overcome their differences and fallen in love. Draco might be at the train station, having changed his ways, waiting to see Harry off with a small token of his affection, like a handkerchief with their initials embroidered on it.
He could meet new witches and wizards from different cultures. For example, he might run into a powerful African wizard who has unique magical abilities related to nature magic. Or he could meet a Japanese witch who specializes in charm - making that is based on ancient Shinto beliefs.
Hermione would definitely be one of his top allies. Her intelligence and knowledge of magic and history would be invaluable in helping Harry rule. She could advise him on laws and policies, especially those related to education and the treatment of non - human magical beings. Ron would also be an ally. His loyalty and his understanding of the wizarding world's less - sophisticated side could help Harry connect with the common wizards.
Hermione would be a likely colleague. She's highly intelligent and could be teaching a subject like Ancient Runes or Arithmancy. Ron could also be there, perhaps as a flying instructor or teaching something related to magical creatures. Neville might be a Herbology professor, given his love and talent for that subject. Luna could be teaching something really unique, like the study of magical creatures that are often overlooked or misunderstood.
Dumbledore could also play a role. Even if he's not alive in the normal sense, his spirit or some form of his magic could guide Harry. He might have known about the connection between Harry and Excalibur all along and left behind some hints or instructions for Harry to follow.
Hermione would definitely be one of his allies. She is smart and resourceful and would be able to help Harry navigate the new political and magical situations he faces as a prince. For example, she could research ancient laws and prophecies related to his new status.
Voldemort and his Death Eaters would still be the main enemies. But now, Harry's guns would pose a new threat to them. They might not be used to dealing with such Muggle - like weapons, even if they are magically enhanced.
Harry might meet some American wizarding families that have long - standing traditions different from the pure - blood families in the UK. These families could introduce Harry to new forms of magic and social norms within the American wizarding community. He could also meet American Muggle - born wizards who have their own unique stories of discovering magic in a different cultural context compared to what he's used to.
Well, Harry could marry one of the veelas. Given their beauty and allure, it would add an interesting dynamic to the story. Their relationship could involve Harry having to deal with the jealousy of other wizards and also the cultural differences of the veelas. Or he could marry a descendant of one of the founders of Hogwarts. This would tie in a lot of Hogwarts history into their relationship, and their wedding could be a grand event that unites different factions within the wizarding community who have ties to the founders.