Just a note as people send me stuff to read over; if you want to have supernaturalish crossovery stuff in your background, sure, okay, that can be fine. Don't however expect it will come up in game at all.
More directly, while it hasn't happened yet, if you invoke such stuff with the expectation to be able to bring it up in game, as far as some, I don't know, mage ally, or vengeful pack of werewolves or something, similarly, that is not really something I want to deal with.
The closest I could possibly picture is some edge case of the Ravnos/Kuei Jin war (though you will have to do some impressive work on how that touches Constantinople, like at all), and even there, at least the other side are also vampires, in their own way.
Yes, for those who have the book, Constantinople has some very, very light crossovery touches. I am not really looking to go especially beyond them as far as anything I have in mind. There's enough going on.