Well, it could be really exciting. Maybe Percy uses his water - controlling powers to help Flash when he's running through some kind of water - based obstacle during a race. Or Flash could use his super speed to help Percy get to a crucial location in the demigod world quickly, like to retrieve a powerful artifact before the bad guys do.
Maybe the Reverse - Flash. He could somehow cross over into the Percy Jackson world. He'd be a difficult adversary for both Percy and Flash. He could use his negative speed force to slow down Percy's reflexes and disrupt Flash's connection to the speed force. Percy would have to rely on his wits and the power of the sea to fight back, while Flash would have to find a way to overcome the Reverse - Flash's power - draining abilities.
Perhaps a rogue demigod who has made a deal with a speed - based meta - human villain from the Flash's world. Together, they plan to take over both the magical realm and Central City. The demigod could use his powers in combination with the meta - human's technology to create some really tough challenges for Percy and the Flash.
No. Percy Jackson is a completely different fictional character with his own unique world and story in the Percy Jackson series. It has no relation to being a Flash fanfiction.
One idea could be that Percy Jackson accidentally gets transported to the Flash's world. He might use his water powers in new and unexpected ways to help the Flash fight villains. For example, creating water barriers or using water currents to slow down fast - moving foes.
One idea could be that the Reverse Flash somehow gets transported to the world of Percy Jackson. He might try to use his speed powers to steal some powerful magical artifacts. The Flash, being the hero, follows him and has to team up with Percy Jackson to stop Reverse Flash. Percy could use his water powers to slow down Reverse Flash while The Flash tries to outrun and capture him.
Well, one idea for this crossover could be that Savitar somehow ends up in the world of Percy Jackson. Maybe he's chasing after some new kind of speed - the speed of a demigod? And Percy has to figure out how to deal with this strange, super - fast being who doesn't seem to follow the normal rules of the Greek myth world.
Another idea is that Sophie Foster from KotLC travels to the Percy Jackson universe. She could be mistaken for a demigod at first. She would have to learn about the Greek gods and monsters. Her telepathic abilities would be really useful in the Percy Jackson world. For example, she could communicate with some of the monsters to find out their plans. And Percy and his friends would help her understand this new and dangerous world.
The Flash could use his super - speed to create distractions while Percy Jackson attacks Reverse Flash with his sword or his powers. For example, The Flash could run around Reverse Flash so fast that it creates a mini - tornado, and while Reverse Flash is disoriented, Percy could strike.
It's a very strange thought. Maybe they are just misinformed or haven't really read either series properly. There's no real similarity between the two that would lead to such a conclusion.