It could be that Arya, being a Targaryen, has some hidden dragon - related powers. The plot might revolve around her trying to master these powers while still holding on to her Stark identity. She could be hunted by those who fear the return of a powerful Targaryen, and she has to use her skills as an assassin to survive and protect her newfound identity.
Maybe Arya discovers she has Targaryen blood. She could then embark on a journey to learn about her new heritage, facing challenges from those who oppose the idea of a Stark - Targaryen connection. She might also have to deal with the power struggles within the Targaryen family remnants.
One possible plot is that Arya's Targaryen lineage is revealed during a time of great unrest in Westeros. This revelation throws the political landscape into chaos. She is then thrust into the middle of a battle for the Iron Throne, not because she desires it, but because her new identity makes her a key player. Along the way, she forms alliances with some unexpected characters and has to make difficult choices between her Stark loyalty and her new Targaryen heritage.
One possible plot twist could be that Arya, being a master of disguise, infiltrates Daenerys' court under a false identity. She initially plans to spy on the Dragon Queen but ends up being impressed by Daenerys' leadership and her quest for justice. This could lead to an unexpected alliance between the two strong - willed women.
I'm not sure if it's a popular fanfic. It could be that it depends on the specific community of 'Game of Thrones' or 'A Song of Ice and Fire' fans. Some might find the concept of Arya being related to the Targaryens interesting, while others may prefer more traditional interpretations. There could also be a lot of other fanfics out there that overshadow it.
Another plot could be centered around Jaqen teaching Arya some of his unique skills. Arya is already a skilled assassin, but Jaqen has his own brand of magic - like his ability to change his face. He could pass on some of this knowledge to Arya, and they could use it to infiltrate enemy strongholds and carry out their plans.
One possible plot could be a story of adventure. Aegon and Arya might meet while on separate quests. For example, Aegon could be seeking to claim his rightful throne and Arya, with her skills of disguise and combat, could be on a mission to avenge her family. They join forces, Aegon learning from Arya's street - smart ways and Arya getting to understand the politics that Aegon is involved in. Their relationship could start as a mutual respect for each other's abilities and grow into a deep friendship as they face numerous challenges together, like battles against common enemies and navigating through the complex web of power in the Seven Kingdoms.
One possible plot could be about the rise of the OC (original character) as a Targaryen king. Maybe he has to fight off rival claimants to the throne, using his unique skills and the power of his dragons. He could also be involved in political intrigue within the Seven Kingdoms, making alliances and enemies as he tries to establish his rule.
One possibility could be that they meet in a war - torn land. Arya has been on her own journey of revenge and Jon has been dealing with the politics of the North. When they reunite, they immediately recognize the changes in each other. Arya is more hardened, while Jon has a new sense of leadership. They share stories of their adventures, and then decide to team up to face a common enemy, like the White Walkers that might still be lurking in the shadows.
The fanfic could be centered around a prophecy. The OC Targaryen has a prophecy that is somehow related to Arianne. They meet, and while trying to figure out the meaning of the prophecy, they fall in love. However, there are forces that don't want the prophecy to be fulfilled, and so they have to fight against these enemies while also exploring their relationship.
The plot could revolve around the orphan boy's discovery of his Targaryen heritage. Maybe he finds an old relic or meets someone who tells him he has Targaryen blood. This could lead him on a journey to claim his rightful place in the world of the Targaryens.
A possible plot could be that a Targaryen descendant discovers they have magical abilities and is sent to Hogwarts. There, they struggle to fit in because of their different magical heritage. They might try to hide their Targaryen traits at first, like their silver hair or their ability to communicate with dragons in some form. Eventually, they could use their unique skills to help Hogwarts in a time of crisis, such as during an attack by dark wizards.
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.