One possible plot could be that Vader, without his suit, is found by Obi - Wan in a hidden location. Obi - Wan, despite their past, decides to help Vader recover. As they interact, they start to understand each other's perspectives better. Maybe they even form an uneasy alliance against a new common enemy in the galaxy.
Perhaps in this fanfiction, suitless Vader has lost his memory. Obi - Wan stumbles upon him and initially is conflicted. But as he spends time with Vader, he realizes that there is a chance for Vader to be redeemed without the influence of the suit and the dark side. They then embark on a journey to find a way to restore Vader's true self while being hunted by the Empire.
Their relationship might start with suspicion. Obi - Wan would be cautious as he's seen Vader's evil deeds. But as Vader is suitless and perhaps more 'human' in a sense, Obi - Wan could start to see the Anakin he once knew. This could lead to a slow rebuilding of trust.
Redemption is a common theme. The OC often serves as a catalyst for Vader's possible redemption, seeing the good in him even without his suit. For example, in many stories, the OC might encourage Vader to turn away from the dark side.
The plot could be about Cody and Obi - Wan getting stranded on a strange planet. They have to rely on each other to survive, learning more about each other's skills and personalities along the way.
One interesting idea could be a female Obi - Wan being a mentor to a younger generation in a different way than the male Obi - Wan. She might use her unique perspective to train Jedi. As for Vader, a fanfiction could explore his relationship with this female Obi - Wan in a more complex way, like they have a grudging respect for each other's skills.
One possible plot could be that they meet during a time - travel incident. Obi - Wan from the Star Wars universe and Kakashi from the Naruto universe find themselves in a neutral dimension. They initially see each other as threats but then realize they have a lot in common as both are highly skilled warriors and mentors. They then team up to find a way back home.
She changes it a great deal. For starters, the gender difference can bring in a new type of relationship. Instead of the more traditional male - male rivalry, there could be an element of attraction or different kind of respect. Also, her way of handling the Force and Jedi teachings might be different, which would affect how Vader reacts to her.
The plot could involve Obi - Wan's hidden influence on the Rebel Alliance. He could be subtly guiding key decisions from the afterlife. For instance, he might nudge Hera Syndulla to take a certain route during a battle that would lead to victory.
A possible plot could be that female Obi - Wan's pregnancy is the result of some sort of experimental force - related phenomenon. This leads to her being ostracized by some in the Jedi Order who fear the unknown. As she progresses through the pregnancy, she discovers new powers within herself and the baby that could change the fate of the galaxy. She then has to fight to prove that her pregnancy and the baby are not a threat but a potential new hope for the Star Wars universe.
There's also 'A New Hope for Vader (Suitless)'. In this fanfiction, the OC is a healer from a far - off planet. When she encounters suitless Vader, she tries to heal not just his physical wounds but also his broken spirit. It's an interesting take on the redemption arc, with the OC playing a crucial role in Vader's journey.
Perhaps Anakin manipulates Obi - Wan's emotions towards him. Anakin could play on their brotherly bond and twist it. He might make Obi - Wan think that by joining the dark side, they can save more lives together. Anakin could also introduce Obi - Wan to some dark side teachings that seem appealing at first glance.