I can't speak for DFRPG specifically, but FATE -- the system which runs the game -- can handle Slayers without much of an issue. The first time I was ever exposed to FATE, I was doing a Fudge hack for a Buffyverse game. (Fudge, FYI, is the system on which FATE is based.) I was almost done to my satisfaction when I discovered FATE, at which point I had to redo it to handle Aspects and Stunts, because they helped out so much with getting things exactly right.
Based on the spoilers it looks like you'd end up with a Slayer if you took the Stunts for super strength, toughness, and recovery. High fighting skills take care of much of the rest. What those don't do, you hit up with appropriate Aspects; this can cover things like flashes of insight from the dreams that come with being a Slayer.
If such a character had an okay Refresh to start, you could take a Stunt for super speed just to round things out.
The Buffyverse is definitely one of the things I intend to run with DFRPG. So I've been reading the teasers with an eye toward that type of thing.