Maybe it's a group of rogue demigods who have turned bad. They see Percy and Annabeth as a threat to their plans of taking over Paris or finding some valuable treasure in the city. These rogue demigods could have unique powers and be very difficult to defeat.
A powerful sorcerer from Greek mythology who has been banished to Paris for centuries could be the villain. He wants to use Percy and Annabeth to break free from his banishment and wreak havoc on both Paris and the world of the gods. He could use his magic to create illusions and traps all over the city to stop our heroes.
The gorgons could be a likely villain. They might be lurking in the catacombs of Paris, waiting to turn Percy and Annabeth into stone. They could be after some powerful magic item that is hidden in the city.
A really good villain could be an ancient Titan who has been awakened from his slumber. He's angry at the gods and wants to take revenge by destroying their favorite demigods, Percy and Annabeth. He has the power to manipulate earth and fire, which makes him a formidable foe.
There might be some new characters introduced in the fanfiction who have different ideals or motives that clash with Annabeth's. These new characters could be the antagonists as they oppose Annabeth, and Percy has to step in. For example, if they think Annabeth's way of handling a certain quest is wrong and Percy believes in her, he'll fight against their ideas.
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.
One possible plot could be that Percy and Annabeth are on a quest in Paris. They might be searching for a lost Greek artifact that has somehow ended up in the city. Along the way, they explore the famous landmarks like the Eiffel Tower and Louvre Museum, all while facing various monsters in disguise among the tourists.
There could be some minor gods or spirits that have been trapped in Tartarus but are still benevolent. These could be beings that Percy and Annabeth come across and who decide to help them in their journey. For example, a spirit that has knowledge of the layout of Tartarus could guide them away from the most dangerous areas. Another possibility is some half - forgotten heroes who have been in Tartarus for a long time and have developed unique skills to survive, and they join forces with Percy and Annabeth.
Obviously, Percy and Annabeth are the main characters. But we could also see some of their demigod friends like Grover. Grover might stop by to check on them and end up getting involved in the babysitting chaos. Maybe Chiron could also make an appearance, giving Percy and Annabeth some advice on how to handle the kids.
Percy might be one. They have a close relationship, and in some fanfictions, he could be more forward with his feelings through flirting.
One likely villain could be Loki. He's always up to no good and loves to play mind games, so a card game would be right up his alley. He might be the one who lures Tony into the game.
It could be Chiron, the centaur. He's always keeping an eye on the campers and would likely be concerned if he caught them up to something. He might give them a stern talking - to and some extra chores as punishment.
In this fanfiction, Natasha Romanoff could be a great ally for Annabeth. Natasha is a skilled spy and fighter, and Annabeth has her own set of combat and survival skills. They could bond over their shared ability to assess situations quickly and make smart decisions in the heat of battle. Also, Natasha might be interested in Annabeth's knowledge of different cultures and mythologies which could potentially help in their missions.