It could be Kronos from the Percy Jackson universe. He could somehow find a way to cross over to Daenerys' world and try to take over. He would see her dragons as a great source of power and want to control them for his own evil purposes. Daenerys and Percy would have to put aside their differences and combine their powers to stop Kronos.
Perhaps it could be a new character created specifically for this fanfic. A powerful sorcerer who wants to rule both the world of Percy Jackson and Daenerys' world. He could be envious of Percy's connection to the gods and Daenerys' dragons. So, he starts causing chaos in both worlds, which forces Percy and Daenerys to team up and find a way to defeat him.
A 'Percy Jackson and Daenerys Targaryen' fanfic could be really exciting. It might start with Percy somehow being transported to the world of Daenerys. Percy, with his demigod powers, could help Daenerys in her conquests. For example, his ability to control water could be useful in battles, like creating floods to defeat her enemies. And Daenerys, with her dragons, could offer Percy a new perspective on power. They could form an alliance and together face new threats that neither of their worlds have seen before.
It could be a villain who wants to merge the Greek and Egyptian mythologies in a chaotic way. Maybe a powerful sorcerer who thinks that by combining the powers of the two worlds without any order, he can become the ultimate ruler.
The main villain could be a power - hungry Atlantean sorcerer who wants to use the power of Atlantis to rule both the underwater world and the surface. He might see Percy as a threat to his plans and try to eliminate him.
Maybe the Reverse - Flash. He could somehow cross over into the Percy Jackson world. He'd be a difficult adversary for both Percy and Flash. He could use his negative speed force to slow down Percy's reflexes and disrupt Flash's connection to the speed force. Percy would have to rely on his wits and the power of the sea to fight back, while Flash would have to find a way to overcome the Reverse - Flash's power - draining abilities.
Loki could be a great villain. He's known for his trickery in Norse mythology. He could manipulate events to set Magnus and Percy against each other. Maybe he wants to cause chaos between the Norse and Greek worlds for his own amusement or to gain more power.
The main villain could be an ancient Titan who was imprisoned long ago but has now found a way to break free. This Titan could be seeking revenge on the gods and demigods alike, and Percy and his friends would have to stop him before he destroys the world as they know it.
An ancient and powerful Titaness who was banished long ago could return. She is angry at both the Greek gods and the current Titans. So she decides to use Percy as a pawn to start a war between the two groups so she can emerge victorious and rule over all.
One great 'oc targaryen daenerys fanfic' is 'A Dragon's Purpose'. It delves deep into Daenerys' character, exploring her journey in a unique way. The author creates an engaging plot that stays true to the essence of Daenerys from the original story.
Maybe some of the more malevolent Greek gods could be antagonists. If Naruto's presence disrupts the balance of power among the gods, some of the jealous or power - hungry ones could try to get rid of him. For example, Hades might see Naruto as a threat to his domain and try to stop him from interfering in the affairs of the underworld.
I really like the fanfic 'Villainous Intent'. It focuses on a group of villains in the Percy Jackson universe coming together to form a new, more powerful threat. The author does a great job of showing their motives and how they interact with each other, which makes it a very engaging read. It's full of action and plot twists that keep you on the edge of your seat.