A really cool plot twist could be Percy finding out he has a long - lost sibling who is on the side of the Titans. This would create a huge internal conflict for Percy as he has to decide between his loyalty to his new - found family and his duty to the Olympians. It would also add a lot of drama and tension to the story as he tries to sway his sibling to the right side.
Perhaps a plot twist where Percy discovers that his own powers are somehow linked to the Bane in a way he never expected. Like his ability to control water is actually being manipulated by the Bane to cause chaos in the ocean realm, which would put him in a really difficult position of having to figure out how to break that connection while still using his powers.
One idea could be Percy going on a quest to find a lost artifact that has the power to restore a dying part of the godly realm. This would involve him facing new monsters and making unexpected alliances.
In some fanfictions, a cool plot twist could be that Percy's connection to Kronos gives him the ability to travel through time. He could use this power to go back and change certain events that led to Kronos' rise or fall. But this also comes with risks, like accidentally changing things for the worse or getting trapped in a different time period.
One interesting plot twist could be that the Staff of Olympus has a hidden power that no one expected. For example, it might be able to reverse time in a small area, which changes the whole dynamic of a battle. Another could be that a character who was thought to be an enemy turns out to be a key ally in the quest related to the Staff. Maybe they were under a spell or had a secret mission all along.
One idea could be Percy having to face a new and powerful monster that is immune to water. This would really challenge his typical combat strategies. Another could be Percy getting lost in a forgotten part of the Underworld and having to find his way out while dealing with various underworldly spirits.
In some fanfictions, a really cool plot twist could be that Percy's Hela - given powers are slowly corrupting him. He starts to see the world in a different, darker way. His friends notice the change in him but don't know the cause at first. As he struggles with this corruption, he has to find a way to either embrace it and use it for good in a new way, or find a way to get rid of it. Also, imagine if Percy discovers that he has a half - brother or sister from Hela's side in the Norse world, and this new relative has a completely different view on life and power, which causes more drama and conflict for Percy.
One idea could be Percy having to deal with a new, powerful enemy that even the Olympians are afraid of. He has to use his unique skills and form new alliances.
A plot twist might be that the oc has to team up with his enemies in the chaos realm to save Olympus. Or perhaps, when he saves Olympus, he accidentally unleashes another greater danger because of his chaotic powers. Maybe the gods who he saves end up betraying him because they fear his power as the son of chaos.
One unique plot twist could be that Percy has hidden powers from his Titan heritage that no one, not even the gods, expected. For example, he might be able to control time in a small way, which is a power not associated with his normal demigod abilities.
One potential plot twist could be that Percy is not actually the son of Helios as he initially thought. Maybe it was a trick by a villain to mislead him and gain access to the power associated with Helios.