A corrupted demigod who has turned to the dark side could also be the main antagonist. This demigod has made a deal with the forces of the House of Hades in exchange for power. They envy Percy and Annabeth's relationship and their status among the demigods, so they try to destroy them. They know all the secrets of the House of Hades and use this knowledge to constantly stay one step ahead of Percy and Annabeth.
The main antagonist could also be an ancient and powerful spirit from the underworld that was sealed away long ago. Percy's presence as the son of Hades accidentally awakens this spirit. The spirit then starts to spread darkness and despair throughout the world of the demigods, and Percy has to find a way to stop it before it's too late.
A group of rogue demigods could be the main antagonists. They might be jealous of the attention the daughter gets because of her parents. So, they plot to make her life difficult and try to drive a wedge between her and the other demigods.
The main antagonist might be an ancient and powerful being that has awakened. This being could see Percy and Annabeth as a potential threat to its plans of taking over the demigod world. So, it uses its influence to have them banished, thinking that without them, it would be easier to conquer. For example, a long - forgotten Titan who has found a way to break free from its prison.
Ares could also be a strong antagonist. He might view the alliance between Annabeth and Zeus as a threat to his own plans for power and chaos. He could try to start wars or create discord to break their alliance and stop whatever they are working towards.
Another plot idea is that there's a rift in the House of Hades that is slowly corrupting the place and they have to find the source of it. Along the way, they face their own inner demons. Annabeth might struggle with her insecurities about being smart enough to solve the mystery, while Percy battles with his fear of not being strong enough to protect Annabeth. As they overcome these personal struggles, they also get closer to solving the problem of the rift.
It could be an old enemy of Percy and Annabeth. Maybe a monster that they thought they had defeated long ago but has come back for revenge. This time, it might target Annabeth's daughter to get back at them.
It could be an old enemy of the Hunters. Maybe a powerful monster that has a grudge against them for past defeats. For instance, a giant who has been waiting for the right time to strike back at the Hunters.
Well, in the House of Hades, they could be put in a situation where they have to make a huge sacrifice for each other. For example, if there was a curse that could only be broken if one of them took on all the pain and suffering of the other. Percy would probably volunteer immediately for Annabeth, and vice versa. This act of selfless love would strengthen their relationship. Also, they could have some light - hearted moments in the middle of all the darkness. Like finding a small, safe haven in the House of Hades where they can just be themselves and have a picnic or something, which would add a new dimension to their relationship.
A possible main antagonist could be Eris, the goddess of strife. She might be jealous of Aphrodite's beauty and power and see Percy as a tool to disrupt Aphrodite's plans. Eris could send her minions to create chaos and conflict between Percy and Aphrodite, trying to make them turn against each other.
Maybe the main antagonist is an old enemy of Percy's who has learned about his newfound ability to hypnotize (even if it was accidental). This enemy decides to use Annabeth as a pawn by hypnotizing her, hoping to lure Percy into a trap and defeat him once and for all.