Perhaps the OC has a special power that is linked to Ventus' past in some way. For example, she can see glimpses of his forgotten memories. This leads to a journey where they explore his past together, while also growing closer and falling in love along the way. They might face challenges from Organization XIII who are interested in the OC's power.
First, make sure their personalities complement each other. Ventus is kind - hearted and brave. So, if the OC is more cautious and analytical, they can balance each other out. For example, Ventus might rush into a fight, and the OC can stop him and think of a better strategy.
I really like the fanfic 'Terra and Ventus: A New Beginning'. It starts with an interesting take on their first meeting. The author does an excellent job of showing their personalities - Terra's determination and Ventus's innocence. It also has some really cool action scenes that are in line with the Kingdom Hearts style.
One idea could be an OC who is a keyblade wielder from a long - lost world. They meet Ventus during his travels and join him on his journey to restore the balance in different worlds. This OC could have a unique keyblade design inspired by their home world's culture.
You can start by looking on fanfiction.net. It has a large collection of Kingdom Hearts fanfictions, and you're likely to find many about Aqua and Ventus there.
In many Kingdom Hearts Union X Ventus fanfictions, the theme of sacrifice is quite prominent. Ventus is often portrayed as someone who is willing to sacrifice for his friends or for the greater good. Another common theme is his struggle with darkness. Since the Kingdom Hearts universe has a strong element of light and dark, Ventus's battle with any encroaching darkness within him, especially as related to his Union X story, is a popular topic. Additionally, the theme of redemption can be found, as Ventus may have to make up for past mistakes or face challenges that test his moral character in the context of Union X.
One great female Ventus fanfiction is 'Ventus's New Path'. It explores Ventus in a unique way as a female character, with her forming new friendships in the Kingdom Hearts universe. It has a really interesting plot that focuses on her growth and how she overcomes challenges.
Perhaps a fanfiction where Ventus has a premonition of a great danger that only he and Vanitas can prevent together. As they journey to stop it, they gradually form an unexpected bond. They learn that their connection is deeper than they initially thought, and this realization changes their relationship throughout the story.
There's a fanfic called 'Aqua's Promise to Ventus'. In this story, Aqua makes a solemn promise to Ventus during a difficult time in their journey. The fanfic is great as it shows the strength of their friendship and how they support each other through hardships. It also has some beautifully described scenes of the Kingdom Hearts world.
One great 'Kingdom Hearts Ventus x Aqua' fanfiction is 'The Bond Beyond Worlds'. It beautifully explores their relationship in a post - game scenario, with lots of emotional exchanges and adventures together.