Well, she could bring a new dynamic to Middle - earth. Her magical abilities might be seen as a great power by some and a threat by others. For example, Sauron's forces might try to capture her to use her magic for their evil purposes. On the other hand, she could help Aragorn in his quest to become king. She could use magic to heal the wounded soldiers or to create illusions to deceive the enemy, just like her father was a master of disguise in the wizarding world.
In this crossover fanfiction, Sirius Black's daughter would be a fish out of water in Middle - earth at first. But as she adapts, she could start to influence the events there. She might teach the hobbits some simple magic tricks, like making things levitate for fun. And when it comes to battles, she could use her Patronus, which could be a powerful symbol of hope in the dark times of Middle - earth, much like the light of the Elven - made weapons. She would also have to learn about the different races and cultures in Middle - earth, and in the process, teach them a bit about the wizarding world she comes from.
She could fit in as a unique and powerful character. Her magic would set her apart. For instance, she could use her wand to perform feats that no one in Middle - earth has seen before. This would make her both a valuable ally and a target. The Fellowship would likely be cautious at first but then realize her good intentions.
One plot idea could be that Sirius Black's daughter discovers her father's true nature and his history through old Hogwarts memories or letters. She might face challenges in the wizarding world because of her father's reputation at first, but then prove herself to be a strong and independent witch. For example, she could be targeted by some Death Eaters' descendants who still hold grudges against Sirius. However, with the help of her friends, she overcomes these difficulties and makes a name for herself in the wizarding community.
The plot could involve the daughter growing up in the wizarding world, facing the challenges of her father's reputation. Maybe she's trying to clear his name while also learning magic at Hogwarts. She could discover some hidden secrets about her father's past and use that knowledge to make a difference in the wizarding community.
One possible plotline could be the daughter's discovery of her true identity. Maybe she was raised in an ordinary Muggle family and suddenly strange things start to happen. She begins to show magical abilities and then discovers she is Sirius Black's daughter. Another plot could involve her search for her father's legacy within the wizarding world, facing both friends and foes along the way.
In some fanfictions, the mother could be a character that was only briefly mentioned in the original series. For example, it could be a witch he knew from his school days at Hogwarts. They reconnected later in life and had a daughter. Or, it could be a new character who has a special connection to the Black family, perhaps a distant relative who got closer to Sirius and they had a child together.
Robin could be a thief - like character. He could steal important items from the enemy, just like Bilbo stole the One Ring from Gollum. His small size and agility would make him good at sneaking around, and he could provide valuable information to the Fellowship by eavesdropping on the enemy's conversations.
One possible plot could be that Eragon somehow gets transported to Middle - earth. He could meet Aragorn and learn from his leadership skills. Maybe they would join forces to fight a common enemy like a new breed of dark creatures created by a combination of Sauron's magic and the dark forces from Eragon's world.
One way it could work is by having Naruto characters in Middle - earth. For example, Naruto with his shadow clone jutsu could be a great asset in the battles against Sauron's forces. He could create clones to confuse the orcs and trolls. Meanwhile, characters from 'Lord of the Rings' could learn from Naruto's ninja skills. Frodo might pick up some stealth techniques from Naruto's training.
The plot could involve Naruto characters in the Middle - earth. Maybe Naruto with his shadow clone jutsu could be used in battles against orcs. For example, he could send a horde of clones to confuse the enemy. And Sasuke with his Sharingan might be able to detect the hidden threats in the dark forests of Middle - earth. It would be a really interesting mix of the two universes' combat styles and magic systems.
A possible crossover would involve Merlin's struggle with the dark forces in Middle - earth. He is used to fighting against Morgana and her minions in Camelot, but in Middle - earth, the enemies are Sauron and his armies. His skills in magic and strategy would be put to the test. He could also form an alliance with Aragorn, sharing his knowledge of leadership and magic to help Aragorn in his quest to become king.