One idea could be Percy having to prove his loyalty to Juno in a series of trials set by the Roman goddess. Another could be Percy discovering a long - lost prophecy related to his new status as Son of Juno and having to fulfill it to save both the Greek and Roman camps. Maybe he has to team up with some unlikely allies, like a group of minor Roman deities who are also trying to gain favor with Juno.
Well, a cool plot could be that Percy, as the Son of Juno, is sent on a quest to retrieve a powerful artifact that was stolen from Juno's temple. Along the way, he has to deal with his own internal conflict of being a Son of Poseidon but now having this new connection to Juno. He might also encounter some old enemies who now see him as a threat because of his new role and the potential power it gives him.
For a 'Percy Jackson Son of Juno Fanfiction', perhaps Percy is forced to participate in a gladiator - style competition in the Roman underworld to earn Juno's forgiveness for some past misdeed. In this competition, he has to face off against other demigods and mythical creatures. This could lead to him forming new friendships and rivalries. He also might learn some long - forgotten secrets about the relationship between the Greek and Roman gods during his time in the underworld.