One idea could be that the male OC is a foreign noble who comes to Winterfell. He has unique fighting skills and Arya is intrigued. They start training together in secret, and as they do, a deep bond forms. This could lead to them going on adventures together, facing threats to the North.
One plot idea could be that Arya and Aegon meet during their travels. Aegon, with his claim to the Iron Throne, is in need of a skilled ally like Arya. She could help him in his political maneuverings in a way that no one else can, using her skills of disguise and stealth. Another could be a story where they are forced to team up against a common enemy, like a new threat rising in the Seven Kingdoms that neither the Starks nor the Targaryens can face alone. They start off distrustful of each other but gradually come to respect and rely on one another.
One plot idea could be Arya and Jon teaming up to uncover a long - hidden family secret. Maybe there's a lost Stark heirloom that has some sort of power, and they have to go on a quest to find it. Another idea is that they are both sent on a diplomatic mission to an unfriendly kingdom, and they have to use their unique skills - Arya's stealth and Jon's leadership - to succeed.
One plot idea could be Arya, in disguise, getting close to Tywin as a servant in King's Landing. She could overhear his plans and use that knowledge to try and undermine the Lannisters from within. Another could be a scenario where Tywin discovers Arya's true identity but decides to keep her alive for some strategic reason, perhaps to use her as a bargaining chip against the Starks. And a third idea might be a story set in an alternate universe where Arya and Tywin form an unlikely alliance against a common, greater threat, like a White Walker invasion in the south before the events at the Wall.
In a fanfiction about Arya Stark being pregnant, she might be in a situation where she has to keep her pregnancy a secret. Maybe she's in a land where her enemies are everywhere, and if they find out, they'll use it against her. She could rely on her skills of disguise and stealth to hide her condition as she tries to find a safe place for herself and her baby.
I really like 'Arya and the Lost Soul'. The OC in this fanfic is a young girl with magical powers who crosses paths with Arya. Their adventures together are full of action and heart - warming moments. Also, 'Arya's Shadow' is worth checking out. The OC is a stealthy figure who becomes Arya's protector in a dangerous world, and the way their relationship develops is fascinating.
One idea could be Arya using her Faceless Man skills to avenge the wrongs done to her family. She could infiltrate the enemies' strongholds unnoticed and take them out one by one.
One idea could be a crossover where Naruto is transported to the world of Westeros. He meets Arya Stark and they team up. Naruto could teach Arya some of his ninja skills, like how to move stealthily like a shinobi. This would enhance Arya's ability to seek revenge on her enemies in a more efficient way.
One plot idea could be Arya having to deal with political intrigue in the North. Maybe there are rival lords who don't fully accept her as Queen and she has to outwit them. Another could be Arya facing an external threat like a new wildling uprising, but she uses her unique skills to handle it. And perhaps she could be on a quest to find a long - lost Stark heirloom that is said to have the power to unite the North even more firmly under her rule.
One idea could be Arya traveling back to the time before her family's downfall. She could use her skills to prevent certain events from happening, like Ned Stark's execution. Maybe she'd change the course of battles by secretly assassinating key enemies.
One idea could be that the OC is a healer from a far - off land who comes to Winterfell. Sansa is initially suspicious but as she sees the OC's kind heart and skills in treating the wounded, she warms up. The OC could also have some secrets about a possible threat coming to the North that they share with Sansa, and together they start to prepare for it.