Another option could be Ares, the Greek god of war. He might be jealous of the newfound alliance between Magnus and Percy. He could start causing problems between the two heroes and their worlds, trying to incite battles and conflicts. He could also team up with some of the more war - loving Norse deities to create more chaos and destruction.
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.