Maybe a character from 'Supernatural', like Castiel, loses his grace and ends up in the world of 'The Originals'. The Originals see him as a strange and powerful being. Klaus might initially want to use him for his own purposes, but they eventually realize they need to work together to stop a group of werewolves that have been enhanced by dark magic and are threatening both the vampire and human populations in the area. And along the way, Castiel learns about the complex family dynamics of the Mikaelsons.
One idea could be that a powerful witch from 'The Originals' world accidentally opens a portal to the 'Supernatural' world. Sam and Dean Winchester are drawn through it and end up in New Orleans. There, they team up with the Mikaelson family to fight a common enemy, like an ancient and powerful demon that is causing chaos in both their worlds.