I think they've done enough with the Scarecrow; I found Batman Begins too hard to follow; the original one with Keaton and Nicholson was better. The Dark Knight was great though, so hopefully the third one is going to be good as well.
But yeah, like I said above, why not have a female villain? We've seen plenty of male villains in these two movies already (Scarecrow, that dude who came after Bruce, Joker, Two-Face, that crooked cop), and there were plenty of female villains in the Batman comics, cartoon, and old series. I'd like to see what they'd do to a female villain in this darker series.
And adding a darker version of Robin and/or Batgirl wouldn't be a bad idea; that was my favorite movie of the old series.