The Joker could be a great antagonist. He could be using some sort of dark magic or other - worldly device that he doesn't fully understand, which attracts Raven's attention. His chaotic nature would also put him at odds with Batman as always, and Raven would have to deal with his unpredictable and often cruel behavior.
One great 'Batman Raven fanfic' is 'The Dark Bond'. It explores a unique relationship between Batman and Raven where they team up to fight a new, powerful villain in Gotham. Their contrasting personalities, Batman's brooding and Raven's mystique, make for an interesting dynamic.
One idea could be that Raven is in danger from a new dark magic threat and Batman, with his detective skills, helps her track down the source. They start to learn from each other, Raven about Batman's human tenacity and Batman about Raven's otherworldly powers.
It can start with a common enemy. They are forced to work together against a threat that targets both Gotham and Raven's world. As they fight side by side, they begin to trust each other.
One possible plot could be that Gotham is being invaded by a dark magical force. Batman, with his detective skills, discovers that the source of this magic has other - dimensional origins. He then seeks out Raven, a powerful empath and half - demon, for her expertise in the magical realm. Together, they embark on a journey through Gotham's seedy underbelly and other dimensions to stop the threat. Raven uses her magic to track the source while Batman uses his combat skills and gadgets to fight off the minions of the dark force.
First, study the characters. Know Batman's no - killing rule, his detective skills, and his dark past. For Raven, understand her powers, her connection to the occult, and her often - withdrawn personality. Then, think of a unique plot. Maybe a new villain that only they can defeat together.
One example could be a fanfic where Batman and Raven team up to stop a powerful villain that neither could defeat alone. Batman respects Raven's magical abilities and Raven admires Batman's strategic mind.
At first, their personalities might clash because Batman is very much a man of logic and order. He plans everything meticulously and is often skeptical. Raven, on the other hand, is more in tune with her emotions (despite trying to keep them in check) and her magic, which can be somewhat unpredictable. Batman may find Raven's reliance on magic hard to trust at first. But as they work together, they start to blend. Batman realizes that Raven's magic gives them an edge in dealing with threats that his gadgets and combat skills alone can't handle. Raven, in turn, sees Batman's discipline and determination and learns from it. She becomes more focused in using her magic in a more calculated way, and they develop a mutual respect that allows their personalities to work in harmony.
There might be a fanfiction where Raven and Batman form an unlikely friendship. Raven, being from a different realm, offers Batman a new perspective on justice and dealing with the criminal underworld. In this story, they could face a common enemy that requires both of their unique skills to defeat.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in 'raven and batman' in the search bar, and you'll get a list of relevant stories. Then, you can read the summaries and reviews to find the ones that seem interesting.