I feel like Aramis and Syaoran would both interact with Elsa in this AU, yeah? She and Aramis would at least know each other off-screen, if nothing else.
Since Sebastian and Ciel are looking for purebloods, they'd be interested in Merlin if they caught wind of what he is. If so, I see potential for laying the foundation for Sebastian and Merlin to talk ~devotion to a master~ after the event ends?
/vaguely offers Elsa to be attacked by any of your vampires /vaguely offers Sebastian to attack any of your humans