Well, a possible storyline is that Rey is lost in a strange part of the galaxy and ends up in a place where Vader still rules. She has to use her wits and the Force to survive and deal with Vader. Maybe she tries to understand his pain and the reasons he turned to the dark side, and in the process, she changes his view a bit. Or perhaps Vader tries to turn Rey to the dark side, seeing her as a powerful addition to his ranks. There are also storylines where they are brought together by a greater power, like an ancient Force - related entity that wants them to either fight or cooperate for some grand purpose.
One great 'Rey meets Darth Vader' fanfiction is 'The Encounter Beyond Time'. It explores what would happen if Rey was somehow transported back in time to Vader's era. Another is 'Vader's Unlikely Apprentice' where Vader takes Rey under his wing in an alternate universe.
In a 'Ahsoka meets Darth Vader' fanfiction, there could be an intense battle. Ahsoka, with her Jedi skills, would be a formidable opponent to Vader. Maybe she tries to redeem him, recalling the Anakin she once knew. Or perhaps Vader sees her as a threat to the Empire and attempts to eliminate her quickly.
Well, in fanfiction, when Darth Vader meets Yoda, it could lead to a battle of ideologies. Vader, having turned to the dark side, represents power and control through the Sith ways. Yoda stands for the Jedi's connection with the Force in a more harmonious and selfless way. They could engage in a Force - powered duel where Vader tries to overpower Yoda with his dark side strength, but Yoda uses his wisdom and experience to counter.
It could explore their relationship by showing the contrast between their current selves. Ahsoka is a free - thinking Jedi - like figure, while Vader is a servant of the Emperor. Their relationship could be one of enmity, with Ahsoka seeing Vader as a fallen hero and Vader seeing her as a nuisance to the Empire.
Perhaps Vader would start with 'You cannot stop me, old one.' Yoda would then calmly say 'Not so, young Skywalker. The Force is with me.' This shows the contrast between Vader's arrogance in the dark side and Yoda's confidence in the light side of the Force. There could also be a moment where Vader asks Yoda why he should return to the light, and Yoda gives a long speech about the true nature of the Force and how the dark side is a path to self - destruction.
Well, in such a fanfiction, R2D2 could use his beeps and whistles to communicate something unexpected to Darth Vader. It could be some long - forgotten information about Anakin Skywalker that makes Vader question his path. And Vader might find himself hesitating to harm the little droid because of this new - found knowledge.
It could be that Darth Vader is intrigued by C3PO's unique design and programming. Since C3PO has been around different planets and species, Vader might try to extract knowledge about various cultures and technologies from him. C3PO, being a protocol droid, would be full of such information. Their meeting could lead to a series of interrogations where C3PO tries to evade Vader's questions while Vader uses his dark side powers to intimidate him.
Well, one possible plot could be that Darth Vader tries to persuade Luke to join the dark side once again. Maybe he shows Luke some 'truths' about the Jedi that he believes are lies, trying to shatter Luke's faith in the Jedi way.