Perhaps Percy turns into a falcon. In this form, he could soar through the skies and spy on his enemies from above. He might use his falcon eyes to find a secret lair of a villain or to track the movements of the Titans. It would also give him a different perspective on the world around him, seeing things from a bird's - eye view.
A snake would be very challenging. Snakes move in a completely different way from humans or demigods. Percy would have to get used to slithering on his belly and seeing the world from a low, sideways perspective. Also, snakes have a different way of sensing their environment, like using their tongues to smell. Percy would need to adapt to this new way of experiencing the world.
One possibility could be that Percy, as a new god, would have to deal with the complex politics of Olympus. He's always been an outsider, and now he'd be thrust into a world of power plays and ancient rivalries. Maybe he'd try to change some of the archaic rules that the gods have held onto for centuries.
One idea could be that Percy turns into a dragon to fight a powerful sea monster. Since he has his connection to water, as a dragon he could breathe a sort of water - based fire or ice that freezes the water around the monster, trapping it.
One possibility is that Percy turns into a falcon. He could then use his new form to spy on his enemies from above. With his sharp eyesight, he might discover hidden lairs or secret meetings. This would give him an advantage in the story's battles or quests.
In a 'Percy Jackson turns evil' fanfiction, he might use his powers against his former friends. For example, his control over water could be used to flood Camp Half - Blood. His knowledge of the demigod world would also be a weapon in his hands to outwit and defeat those who oppose him.
Well, in the fanfiction, if Percy turns into a baby, he would probably lose all his skills and knowledge at first. But he might still have some instinctual connection to water. Maybe his friends like Annabeth would have to take care of him. She'd be really protective and try to find a way to turn him back.
In the 'Percy Jackson turns into a cat' fanfiction, he could end up in some really interesting situations. For example, he might be chased by some of the magical creatures at Camp Half - Blood that don't recognize him as Percy. His senses would be heightened as a cat, so he could detect things like danger or hidden treasures more easily. Also, it would be hilarious to see how Annabeth reacts when she first sees Percy as a cat. She might be confused at first but then try to figure out how to turn him back.
Well, his friends' reactions would be quite diverse. Annabeth might see it as an interesting new development. She could be a bit protective, making sure that Percy, as a girl, isn't taken advantage of. Grover would likely be his goofy self, but also offer his support. He might make some silly jokes at first to lighten the mood. Other friends at Camp Half - Blood would also have different reactions. Some might be curious, some might be a bit hesitant, but overall, they'd probably accept Percy in this new form because they know it's still Percy deep down, with the same brave heart.
One possible plot could be Percy waking up as a girl and having to deal with the new physical and social aspects in Camp Half - Blood. Maybe the other campers are initially confused, but then they see that Percy (now a girl) still has the same heroic spirit and skills. She could go on a quest where her new female form gives her a different perspective on things, like how she interacts with female monsters or how she is treated by male gods differently.