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.
One common theme is mentorship. Batman often takes on a mentorship role towards Kid Flash, guiding him on how to be a better hero. Another theme is the contrast between their powers and personalities. Batman's dark and brooding nature against Kid Flash's more light - hearted and speedy style.
The Riddler could be a great main antagonist. He could use his intellect to create puzzles and traps that not only challenge Batman's detective skills but also the Flash's speed. For example, he could set up a series of time - locked puzzles where the Flash has to use his speed in a very specific way at specific times to unlock them, while Batman has to figure out the overall pattern of the puzzles.
One of the most popular pairings is Flash and Batman themselves in a platonic partnership. Fans love to see their different personalities and skills mesh together. Another popular pairing could be Flash with one of Batman's sidekicks, like Robin. The contrast between Flash's super - speed and Robin's acrobatics and detective skills can make for an interesting dynamic.