Harry may decide to have a more intuitive magic system. Instead of relying on wands, which can be lost or broken, the magical beings on his planet could use their minds and emotions to cast spells. The stronger the emotion or intention, the more powerful the spell. And to prevent misuse, there could be a natural balance in the planet's magic. If someone tries to use dark magic, the planet itself could resist and weaken the spell, teaching the user that dark magic is not welcome.
It could be a planet where magic is more powerful and less restricted. Harry, having faced so many limitations in the wizarding world he knew, would create a place where magic can be used freely for good. There could be no dark wizards or threats like Voldemort, and wizards could focus on exploring and expanding their magical abilities. For example, they could create new spells without the fear of them being used for evil.
Perhaps he discovers the planet during one of his adventures. He could be using a new spell that accidentally transports him to this uncharted planet, and then through some series of events, he becomes the rightful owner. For example, he might save the planet from a great evil and as a reward, the planet's native beings make him the owner.
Well, perhaps it's a planet with its own system of magic. It might have a core made of pure magic essence which gives power to all the magic on the planet. And there could be different regions, like one area filled with floating islands where powerful wizards live.
He could have found a magical artifact. There are many artifacts in the Harry Potter world that have strange and powerful abilities. It could be that this artifact has the power to create clones. He figures out how to use it, perhaps with the help of Hermione who is good at researching such things. And then he starts creating his clone army.
He would first need to understand the environment. If the planet has different gravity, he'd have to adjust his movements. For instance, if it has low gravity, he might accidentally float around until he gets used to it.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
One idea could be that Harry discovers an ancient spell book in the Room of Requirement that has the incantations for creating a magic railgun. He might use it to defend Hogwarts against a new, powerful dark force that is immune to traditional magic spells.
Well, perhaps Harry uses his magic to shape the planet. He might create unique magical creatures and ecosystems. For example, he could make a forest where the trees are made of living fire that doesn't burn but gives warmth and light.
It could be Harry Potter himself, as he might be the one to explore the other planet. Hermione would also likely be there, using her vast knowledge to understand the new world. Ron might accompany them as well, providing his loyalty and humor.
Well, an apocalypse in Harry Potter fanfiction can change the magic system in multiple ways. Firstly, the availability of magical resources might change. For example, if there are no more potion ingredients growing in the wild, wizards have to find alternatives or create synthetic ones. Secondly, the way magic is taught and learned could be different. There might be a more urgent need for self - taught magic as Hogwarts could be in ruins. And finally, the very nature of some spells could be altered. Dark magic might become more prevalent and the boundaries between dark and light magic could become blurred as everyone is just trying to survive.