A cool plot could be that Percy and Mrs. O'Leary time - travel back to an era when the gods were just starting to interact with mortals. They have to navigate this strange time, and Mrs. O'Leary's presence causes some unexpected reactions from the early demigods and gods. Percy has to figure out how to get them back to their own time while also dealing with the chaos they've created.
One idea could be that Mrs. O'Leary gets lost in the mortal world and Percy has to find her without using his demigod powers. It would be a fun adventure as he has to rely on his wits and the help of some mortal friends.
Another plot could be that Mrs. O'Leary gets lost in the mortal world, and Percy has to find her. Along the way, he discovers that she has made some interesting mortal friends who are in danger because of her association with the demigod world. Percy then has to not only find Mrs. O'Leary but also protect her new friends from the monsters that are after them.
You can start by looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just search for 'Percy Jackson Mrs O'Leary' on their search bars and you'll likely find a variety of stories.
A powerful Titan who was previously imprisoned but has now escaped could be a great antagonist. This Titan could have a grudge against Percy for his role in previous battles and sees Mrs. O'Leary as a way to get to him. He could use his own dark magic to try and control Mrs. O'Leary or turn her against Percy.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
No. Writing sexual fanfic about someone is inappropriate and unethical as it violates a person's privacy and dignity.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.
One idea could be Percy having to make a deal with a minor underworld god to save Annabeth from a new and strange curse that's slowly draining her life force. He has to complete a series of tasks in the underworld that test his loyalty and courage more than ever before.
Well, perhaps the shield was once owned by a powerful demigod from the past. In the fanfiction, Percy starts having strange dreams about this demigod and the shield's true origin. As he investigates, he realizes that the shield holds the key to defeating a new, emerging evil that threatens not just Camp Half - Blood, but the entire world of gods and mortals. So, he has to figure out how to unlock the shield's full potential.