Well, one key element could be magic. Since it's in the Harry Potter universe, magic would play a huge role in creating his own world. Another might be the characters he'd bring into this new world. He could take his friends like Ron and Hermione and give them new adventures. Also, the settings would be important. Maybe he creates a new Hogwarts-like school but with different rules and mysteries.
Well, if Harry Potter creates his own magic in fanfiction, it could lead to him being either celebrated or feared. On one hand, his new magic could solve many of the problems the wizarding world faces. But on the other hand, the Ministry of Magic might see it as a threat to their control. They could try to regulate or even suppress his new magical creations.
He might choose a phoenix feather because it represents rebirth and magic. And for the wood, perhaps he'd pick holly, which is often associated with protection in the wizarding world.
Harry might first decide on the values of his world. Is it a world that values equality above all? Based on that, he'd then start to create institutions. He could build a magical council that enforces these values. He would also need to think about the inhabitants. Would he populate it with new species or just bring in his old friends? He could use his creativity to design unique magical creatures that are different from those in the original Potter world. And of course, he'd have to establish a system of magic that is distinct yet still related to what he already knows.
Harry Potter himself is still very popular as he is the one creating the world. People are interested in seeing how he uses his knowledge and power to shape this new place.
One idea could be that Naruto creates his own village as a place of true peace and equality. He invites all those who have been marginalized or ostracized from other villages to join. His village would have unique training methods that combine all the ninja arts he has learned from different places, like the shadow clone jutsu combined with Sand Village's puppet techniques for a new form of combat.
One idea could be that Naruto creates his sword using his chakra. He might infuse it with the power of his Nine - Tails chakra, making the sword have a unique orange glow. This sword could have the ability to cut through any jutsu, like a hot knife through butter.
One idea could be that Harry creates a school for magical creatures. He realizes that there are so many unique and often misunderstood creatures in the wizarding world, and he wants to give them a place to learn and grow. This school could teach them how to interact with wizards in a positive way, as well as develop their own magical abilities.
Perhaps his father's old wand. It has a connection to his family and his heritage. Since it's a wand, it already has a certain amount of magical energy, so it could potentially be a suitable vessel for a Horcrux. And it would be a very interesting twist on the concept of a Horcrux as wands are not a typical choice.
His new taijutsu could change the balance of power in battles. If it's a very powerful form of taijutsu, it could give his village an edge in conflicts. Enemies would have to find new ways to counter his moves, which might lead to a shift in military strategies across the ninja world.