One common theme is the contrast between their powers. Batman is a normal human with great skills and gadgets, while The Flash has super - speed. Fanfictions often explore how they bridge this gap to work together effectively.
The Reverse - Flash could be a great main villain. He could travel to Gotham to cause chaos and draw out the Flash. His time - manipulation abilities would pose a huge threat not only to the Flash but also to Batman's carefully planned strategies in Gotham. Batman would have to adapt to dealing with a villain who can move at super - speed and mess with time.
A great story idea might be that a common enemy, like a super - intelligent villain who can manipulate time, targets both the Flash's Central City and Batman's Gotham. This forces them to team up. The Flash could use his speed to gather information quickly, while Batman uses his intellect and detective skills to analyze it. They'd have to combine their strengths to take down this powerful foe. For example, Batman could create a device to slow down the villain's time - manipulation powers, and the Flash could deliver it at super - speed.
Another good one is 'Allies in the Dark'. In this story, The Flash finds himself in Gotham and teams up with Batman to take on a new threat that is lurking in the shadows. The story delves deep into their relationship as heroes from different worlds coming together to protect the city. It shows how they respect each other's abilities and learn from one another.
A cool plot might be that a new meta - human emerges who has the power to steal the abilities of other heroes. This meta - human targets the Flash first, leaving him powerless. Batman then has to take the Flash under his wing and train him in more traditional combat skills while they figure out a way to get the Flash's powers back. Along the way, they discover that the mastermind behind this meta - human is an old enemy of Batman who wants to use the Flash's powers to carry out a grand scheme in Gotham.