In fanfiction, Percy's power to control all liquids is unique because it offers a whole new dimension of interaction with the world. He can use liquids as a means of transportation by creating a sort of liquid pathway. This is something that no other character with normal powers can do. Also, his power has implications in terms of stealth. He can use liquids to camouflage himself or others. For instance, he can make someone blend in with a body of water or a pool of oil. Moreover, his power over liquids gives him an almost unlimited source of creativity. He can come up with unique solutions to problems by using the properties of different liquids, like using the viscosity of a thick liquid to trap an enemy.
He could use it for various things. For example, he could create barriers of water to protect himself or others. He might also be able to manipulate the flow of liquids in a chemical experiment in the fanfiction world, leading to new discoveries. Another application could be in firefighting, where he can control any liquid and use it to extinguish fires more effectively.
Some fanfictions might suggest that Percy's control over blood is a new power he discovers while on a special quest. He might find out that his powers are not just limited to water bodies but can also extend to the fluids within living beings. He could start to experiment with this new - found ability, and at first, it might be a bit clumsy. But as he gets more practice, he could use it for various things, like healing (by manipulating the blood's flow to injured areas) or even in combat (by making an opponent's blood act in a way that disables them, like making it rush to their head and cause dizziness).
Well, in some fanfictions, it might be related to his demigod powers. Since he has a connection to the water element, blood being a liquid could potentially be manipulated in a way similar to how he controls water. Maybe he can sense the water content in blood and use his will to move or influence it.
Well, in some fanfictions, Percy might control ice through a new power bestowed upon him by some mysterious god or entity. Maybe he discovers he has the ability during a quest when he's in a really cold and magical place.
Well, it could make the plot full of twists. For example, he might use his poison - controlling ability to defeat enemies in unexpected ways, like making the poison act as a sort of weapon that can target specific foes. This would create a lot of suspense as the other characters might not be aware of the full extent of his power.
Another way could be via some sort of magical artifact. For example, he discovers an ancient amulet that has the power to enhance his natural abilities. When he touches it, it starts to merge with his essence, gradually making him stronger and endowing him with new powers like the ability to control different elements related to water, not just manipulate it.
In some fanfictions, Percy might discover his ice - controlling ability during a life - threatening situation. For example, he could be in a battle against a powerful fire - using enemy, and in a moment of desperation, he unknowingly taps into the power to control ice to counter the heat.
Well, in some fanfictions, Percy might use his water - controlling abilities. Since poison can be in liquid form, he could manipulate the water within the poison, perhaps separating out the harmful components. Another way could be through his demigod powers, which might give him an innate resistance or control over harmful substances like poison.
In some fanfictions, Percy might gain power in Tartarus through facing and overcoming numerous deadly challenges. For example, by defeating powerful monsters that inhabit Tartarus, he could absorb some of their essence or gain knowledge from them which then transforms into power. Another way could be through making pacts with certain entities in Tartarus in exchange for power, although this would likely come with a price.
Percy may also try to find loopholes. Given his cleverness and his experience in dealing with the complex world of gods and prophecies, he would look for ways to work around the control of fate. He could use his relationships with other characters, like Annabeth's wisdom or Grover's loyalty, to help him figure out how to gain more control over his own destiny in the fanfiction.