Thinking now of the crossovers, of course. Maybe a dimensional rift, Buffy falls into the Sanctuaryverse. Helen would get so many Ashley feels, that bit would be painful, but Tesla's panic would be hilarious.
By the way, rambling here, but one of Tesla's plans for world domination was, within the parameters of the show's reality, downright brilliant: he set up a detox clinic in Mexico for the heirs to the wealthy and powerful (although in a global world, I hasten to add, you definitely need to have subsidiary clinics in Europe and Asia), and besides curing addiction in a handful of days (neat trick), he gave them slow-release vampire DNA (don't ask, the biology of the show is worse than anime physics), which would make suddenly vampirize them in about thirty years.
That's positively brilliant! Given global inequality and the general inheritability of wealth and power, you just need to pre-vampirize the next generation of powerful people (while they are young, vulnerable, and less well-protected than they'll ever be), and then wait a few decades sipping wine until suddenly the world is, again, run by vampires.
(I hasten to add, Tesla's blind spot is his idolization of the old vampire race; vampirized global leaders would be no more likely to pay heed to Tesla than non-vampirized ones, and probably less so. In fact, it's not until Tesla is nearly killed by the resurrected Vampire Queen that he sort-of gets over his pro-vampire chauvinism, going back to his more natural essence of pro-Teslanism.)