Obviously Naruto himself would be a main character as the source of the clones. Maybe Sasuke could also be involved, either as an ally or an antagonist in relation to the clone army.
Obviously Naruto himself is always popular as he is the one who creates the shadow clone army. Sasuke also often appears in these fanfictions, sometimes as an ally and sometimes as an antagonist to Naruto's use of the clones. Kakashi is another character that is frequently included. His wisdom and experience can play important roles in stories involving the shadow clone army, like training Naruto to better control his clones or using his own knowledge to strategize with the clone - based forces.
One idea could be the clones developing their own personalities over time. Since they are all part of Naruto, they might start to have different outlooks on life. Another could be using the clone army to fight a new and powerful enemy that the regular shinobi forces can't handle alone.
One idea could be the shadow clone army being used in a completely new and unexpected mission. For example, they are sent to a parallel universe to retrieve a powerful and lost ninja art. Another plot could involve the clones developing their own unique personalities over time and causing chaos within the original Naruto's plans. They might start to make decisions on their own, like deciding to help the enemy for some strange reason.
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.
In a fanfiction about the droid army, you could write about a secret mission. The droid army is sent to a far - flung planet to retrieve an ancient artifact. But the planet has its own defenses and the droids have to overcome many challenges. The story could detail their battles against the native creatures and the traps they encounter along the way.
Well, he might use his powerful jutsu like the Rasengan. He could also summon his toads to help him out in the battle against the army. Maybe he uses his shadow clone technique to confuse the enemy and then strike them when they're vulnerable.
It could also change his relationships. His friends might be worried about this new and rather extreme power he has. They might question his motives and whether this is the right way to deal with the problems in the wizarding world. On the other hand, his enemies would definitely be more cautious around him.
In the 'Harry Potter makes a clone army fanfiction', perhaps Harry creates the clone army to protect Hogwarts. He might use some ancient magic spells he discovers in the library. These clones could be modeled after the greatest wizards in history, like Merlin. They would bring a whole new level of defense to the school against the dark forces.
Well, he might start by using his charisma. Naruto has always been good at making friends. He could reach out to old allies from his village, like Shikamaru and Choji. Also, he might convince some of the outcast ninja who have been mistreated in other villages to join him. His reputation as a hero who saved the world would surely attract a lot of people.