A powerful Titan who was once imprisoned by Zeus and now wants to destroy his legacy. This Titan could be seeking revenge for his long imprisonment and sees the legacy as a symbol of Zeus's power that he wants to crush.
Maybe in the fanfiction, Zeus has left a hidden prophecy about his legacy that Percy discovers. This prophecy could involve a great power that is locked away and can only be accessed by following a series of dangerous quests. Percy then has to navigate through various challenges, including dealing with other gods who are either jealous or afraid of what this legacy might bring.
In this kind of fanfiction, Percy might challenge Zeus over issues like the unfair treatment of demigods. For example, Zeus could be overly harsh on demigods who make minor mistakes while the gods themselves get away with much worse. Percy, being a hero, would stand up for his fellow demigods.
Another possibility is that Percy discovers some long - forgotten weakness of Zeus. In the world of fanfiction, the author could create a scenario where Percy stumbles upon an ancient prophecy or a hidden secret about Zeus. This knowledge could give Percy the upper hand. He could then use his wits and his powers to exploit this weakness and defeat Zeus.
One interesting fanfiction could be about Percy Jackson's adventures that are more closely tied to his relation as Zeus's grandson. Maybe he discovers some hidden powers that are passed down only through the Zeus lineage. He could face challenges where his Zeus - related powers are both a blessing and a curse, like when he has to control excessive lightning powers during a battle with a powerful monster.
Another reason for the allure is the chance to re - imagine the characters in the context of Venus' influence. The demigods might develop new skills or attitudes. They could become more empathetic due to the influence of Venus' love - related powers. This can lead to interesting character arcs and development that we haven't seen in the original series. Also, new antagonists could emerge, those who seek to corrupt the power of Venus for their own evil purposes.
One idea could be Percy having to mediate between Zeus and Hades during a major feud. Since he's the son of both in this fanfiction, he has a unique connection to them. He could use his powers and his knowledge of both of their domains to try and broker peace.
For the demigods, things could change a lot too. Percy is a champion of the demigods, and his victory over Zeus might mean better treatment for demigods. They could be more respected and have more rights in the world of gods and mortals. However, it could also make them targets of those who are against Percy's actions.
In some fanfictions, Zeus interacts with Percy in a hostile way at first. He sees Percy as a threat to his power and the order of Olympus. For example, he might try to limit Percy's abilities or send challenges to prove Percy's unworthiness.
One great fanfiction idea could be a story where Zeus, Poseidon and Hades have to team up to face a common threat that is even more powerful than them. Maybe a primordial being awakens and they must put aside their differences. In this story, Percy could be the mediator between them, using his unique position as the son of Poseidon to try and keep the peace among the three powerful gods.