One idea could be that the demigods from Percy Jackson's world have to team up with the vampires from Twilight to fight a common enemy, like an ancient and powerful dark force. Maybe a primordial being in the Percy Jackson universe has awakened and its influence is spreading, causing chaos even in the Twilight world, so they must combine their powers.
A third idea is that there is a prophecy in both worlds that foretells of a union between a demigod and a vampire. A character from each side, say Annabeth from Percy Jackson and Edward from Twilight, are initially reluctant but then realize they must work together to fulfill the prophecy and save both of their worlds from a looming disaster.
One idea could be that the demigods from Percy Jackson's world end up in Forks for some reason. Maybe they are on a quest that leads them there. And they cross paths with the vampires and werewolves from Twilight. For example, Percy could use his water powers to fend off some danger that the Twilight characters are facing, like a wildfire started by a vampire's power going out of control. This would create an interesting dynamic as the two different mythologies collide.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.
Another plot could involve Percy's friends getting accidentally transported to a different realm in the Percy Jackson universe. They have to rely on their friendship and the skills they've learned from Percy to survive and find their way back. For example, they might end up in the Underworld and have to navigate past various challenges without Percy at first.
One idea could be a mortal accidentally stumbling into Camp Half - Blood and having to adapt quickly to the world of demigods. Maybe they have some hidden power that only reveals itself in this new environment.
One idea could be to create a new quest within the game world. For example, Percy and his friends have to retrieve a powerful artifact that was stolen by a new type of monster. This could involve exploring new areas of the game, like a hidden underwater city that has never been discovered before. Another plot could be a tournament in the game where demigods from different cabins compete against each other in various challenges. This would show off their different skills and powers. A third idea is that there's a glitch in the game that's causing real - world problems in the Percy Jackson universe, and they have to figure out how to fix it before it's too late.
One plot idea could be Gaea's redemption. In the original series, she was a major villain, but in a fanfic, she could be given a chance at redemption. Maybe Percy and his friends discover that Gaea was being controlled by a more powerful and malevolent force all along, and they have to work with her to defeat this greater evil.
In a Percy Jackson Tartarus fanfic, a great plot could be centered around Tartarus corrupting the very essence of Percy's powers. He starts to lose control of his water - based abilities and instead they become tainted with the darkness of Tartarus. Percy then has to go on a journey within Tartarus to find a way to cleanse his powers, all while being hunted by the most dangerous creatures that lurk in the abyss.
A really interesting plot would be if Percy lost his memory and ended up in a mortal town. He has to figure out who he is on his own while still being hunted by some old enemies. The gods are aware of his situation but can't interfere directly because of some ancient rule. So Percy has to rely on his instincts and any small clues he can find about his past to get back to his normal self and his life at Camp Half - Blood.
One idea could be that Hylla and Percy team up to rescue a group of demigods captured by an ancient monster. They use their combined powers and wits to outsmart the beast.
A possible plot could involve Percy being sent to Atlantis on a mission by the gods. In Atlantis, he uncovers a conspiracy among some of the Atlantean nobles who are planning to overthrow the current ruling family and take over the power. Percy then has to decide whether to get involved in the internal politics of Atlantis or just focus on his original mission. Along the way, he also learns about the unique culture and magic of Atlantis, which is different from what he's used to in the surface world.