I didn't watch Buffy regularly on TV (nor do I watch the reruns) because there are only so many hours in the day. But this is an intriguing story line. I, for one, would be disappointed to see it stop. I vote to continue!
Lois is a Slayer? Actually, that's a logical career move for her. I like it. And her inability to get along with her fellow Slayers is totally consistent with her pre-Clark personality. Plus, it adds more opportunities for conflict between Buffy and Lois.
You haven't even hinted at this world's Clark. Is he Kryptonian? Is he a vampire? Is he even alive? Will Lois kiss him or dismember him at their first meeting?
And, like others, I wonder if Superman will participate in the 'great vampire roundup.' Can he kill a being whose life depends on the destruction of the innocent? I'm glad I don't have to make that moral choice, but the way you've set this up, our Clark will have to either kill vampires and/or demons or allow the deaths of the helpless.
And what happens if one or more vampires do decide to negotiate in good faith? What would that do to the whole community of vampires? I know, there really isn't one vampire nation, but I bet there's an uber-vampire out there somewhere who wouldn't want his or her authority challenged by some weak-willed peaceniks who'd rather live in harmony with their food source than just consume them.
Ooh, I can see some neat plot turns coming! And I hope that's exactly what they do (keep coming, that is).