I think Poseidon is a must - he's Percy's father, after all. Then there's Zeus, who likes to be in control and would want to see this baby who could grow to be a significant force. Apollo might come just for the fun of it, being the god of many things including prophecy, he might have some insights. And Artemis, as the goddess of the hunt, could also be there to see what all the fuss is about regarding baby Percy.
Obviously, Percy is one of the main characters. And then there would be various gods like Zeus, Poseidon, Athena, etc. These gods play important roles in Percy's world and their interactions with him would be central to the story.
Poseidon would probably be one of the main gods watching. Since Percy is his son, he would have a strong interest in his well - being during the fight.
Aphrodite could also be one of the spying gods. Since she loves meddling in love affairs, and Percy and Annabeth's relationship is quite famous among the demigods. She might be spying to see how their relationship develops and perhaps add a little nudge here and there if she feels like it.
Obviously, Percy would be one of the main characters. And of course, the gods would play important roles too. Gods like Zeus, Poseidon, and Athena are likely to be there. Percy's friends from the demigod world, like Annabeth, might also make appearances.
Obviously, Percy Jackson himself. He's the one doing the pranking. Then, there would probably be the gods like Zeus, Poseidon, and Athena who are the targets of his pranks. His friends like Annabeth might also be involved, either helping Percy with the pranks or trying to stop him when they think he's going too far.
Percy is the central figure for sure. The gods, like Hades, might be there too if the story involves issues related to the Underworld. Grover could also play a role. He is Percy's friend and might be there to support Percy when he yells at the gods, or his experiences with the gods could be part of what drives Percy to yell. Chiron, the centaur, could also be a character as he is a mentor to Percy and could have some influence on the situation.
Well, it could be that the gods, being curious about this special baby Percy, come to visit him. Maybe they bring gifts like in the story of baby Hercules. They might see his potential and start to argue among themselves about his future role in the world of demigods.
Athena might also be a good candidate. Although she and Poseidon have their differences, she could see Percy's potential and befriend him. She could offer him wisdom and strategic advice for his various adventures.
Odin is very likely to be in Percy's fanfiction. His wisdom and his role as the all - seeing god could make for some interesting plot points. Thor is also a mainstay, with his superhuman strength and his connection to thunder. Loki is another one. His mischievous nature could create all kinds of trouble and twists in the story. He could be an antagonist or an unexpected ally to Percy.
Percy is bound to be central. Alongside him, some new characters could be introduced. Maybe there are demigods who are against Percy's return because they see him as a threat to their own status. But the main gods would still be major players. They could be portrayed in different lights, some being more desperate to get Percy back than others, which could lead to interesting power struggles among them.