Stephanie Brown is many things: Tim Drake's girlfriend, the daughter of the Cluemaster, very briefly the 4th Robin, and more. Her life reads a little bit like a soap opera character's, so apologies in advance, and be warned that some recent (within the last year) comic book plot will be discussed.
Since he was in jail for most of her childhood, Stephanie knew her father was a criminal, but when he finally returned home he claimed to be done with crime. But Stephanie soon discovered that he was up to his old tricks, so she whipped up a spooky purple costume, called herself the Spoiler, and stopped her father. At first she would only become Spoiler to stop the Cluemaster, but eventually she grew to like fighting crime.
Some stuff happened at this point, she got pregnant, gave the baby up for adoption, Gotham was mostly destroyed by an earthquake, she and Tim Drake started dating even though she only knew him as Robin, she started getting trained by Batman and Batgirl to become a better crime fighter, and her father died. Pretty rough stretch, huh?
After all this, Tim was forced to quit fighting crime, so Stephanie made herself a Robin costume and auditioned to be Tim's replacement. Reluctantly, Batman agreed and put her through more rigorous training. Eventually though, Batman decided she was too disobedient and told her to quit. Disobeying even that, Stephanie decided to prove herself by stealing one of Batman's plans to take down organized crime in Gotham. She invited all of the heads of the families to a meeting where Matches Malone would sieze power and take over all crime in Gotham. Unfortunately, no one told her who Matches Malone was, and the botched meeeting lead to an all out gang war that would eventually involve Black Mask beating Stephanie to death.
Strangely, Batman never made a memorial to Stephanie, even though Jason Todd's costume has been prominently displayed in the Batcave since his death. This outraged fans enough to start Project Girl Wonder, demanding a memorial for Stephanie. Recently, this was all explained away by bringing Stephanie back from the dead. Leslie Tompkins, the doctor who pronounced her dead and a friend of Bruce/critic of Batman, had faked Stephanie's death to try to get Bruce to see the danger in his methods. Batman never built the memorial because he suspected this all along. And that brings us more or less up to date on her.
Pros: She's a fun character, and there's certainly a lot of story to draw from if you're so inclined.
Cons: She's mostly built around being Tim Drake's girlfriend and the Cluemaster's daughter, so without both of those, it's tough to include her. And there's no way Tim Drake and the Cluemaster will be in a Batman movie any time soon.
How I'd do it: I think you'd need an entire movie to establish some kind of Robin before introducing Spoiler, so there's no way I'd put Stephanie in the next movie.
Casting Guesses: Another teenager type role that I think you'd need to go through lots of casting to find someone for. Either that, or Dakota Fanning might actually be about that age by now, and at least as a little kid she could act. Haven't seen her lately though, who knows.
Verdict: We're more likely to see a Prius Batmobile.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.