Well, obviously the Flash and Arrow themselves are very popular. Their dynamic, with the Flash's light - heartedness and super - speed contrasted with Arrow's brooding and combat skills, is really appealing. But also, characters like Felicity Smoak are often popular. She's smart and has great chemistry with both heroes, so she often plays an important role in fanfictions.
In some fanfictions, there could be a storyline where the Flash and Arrow are transported to an alternate dimension. In this new world, they have to find their way back home while also dealing with the threats in that dimension. It could involve them meeting alternate versions of themselves or their friends, which would be really cool to explore.
Barry Allen is a very interesting character to write about. His super - speed gives a lot of scope for creative writing. You can explore how he experiences time differently, or how he uses his speed in unique ways to solve problems. Oliver Queen is also great. His dual identity as a playboy billionaire and a vigilante archer provides a lot of depth. Writers can delve into his internal struggles, like his fight against his dark side and his journey to become a better hero.
One popular storyline is the crossovers where Flash and Arrow team up against a common, powerful enemy that neither could face alone. For example, a super - powered villain who can manipulate time, forcing them to combine their skills and resources. Another common plot is the exploration of their friendship. Flash's light - heartedness contrasting with Arrow's brooding nature often leads to interesting character dynamics.
Green Arrow isn't typically portrayed as an asshole. He's often a hero with a strong sense of justice, but like any character, he has his flaws and moments of difficult choices.