In a 'Percy Jackson Son of Juno Fanfiction', the Triumvirate could also be antagonists. They might view Percy's new status as a way to gain more power or as a threat to their own plans. They could send their minions after Percy, like the Eidolons or some powerful monsters, to either capture or kill him so that they can continue their control over the demigod world.
Some of the more vengeful Roman gods who are jealous of Juno's new 'son' could be antagonists. For example, a god who feels that Percy is getting special treatment from Juno might try to sabotage his quests or turn other demigods against him. Also, some of the old Titans who are still lurking in the shadows might see Percy as a threat to their potential comeback and try to eliminate him.
The Giants could be great antagonists. Since Percy is now connected to Juno, they might see him as a key figure to target in their war against the gods. They could try to use his new identity against him and the other demigods.