Talia, daughter of Batman Begins villain Ra's al Ghul, is one of Batman's great love interests. Just Ra's al Ghul was designed as a James Bond-style enemy for Batman, Talia is not unlike a Bond girl. She's beautiful, exotic, deadly, and hopelessly attracted to Batman. In the comic books, Ra's al Ghul is so impressed with Batman in their initial encounters, he decides that Batman is worthy of his daughter's hand in marriage. Though Batman turned down the offer, recent comic book stories have involved Damian Wayne, conceived during Batman and Talia's time together.
Talia has had a number of different roles in the DC comics universe: Batman's love interest, replacement for Ra's al Ghul, head of Lex Luthor's company LexCorp, leader of an organization of supervillains, and mother to Damian. She's a skilled martial artist, a ruthless businesswoman, a brilliant strategist, and irresistibly attractive.
Pros: Bringing in Ra's al Ghul's daughter would make a nice callback to Batman Begins, Talia would make a nice replacement for Rachel Dawes as Bruce/Batman's love interest, and her lack of supernatural abilities fits the style of the movies.
Cons: She's not that well known outside of comic books. She had a few appearances in the Batman animated series but wasn't on the Adam West TV series or any previous movie versions. So it would be difficult to get movie fans excited about Talia's appearance. There's also a problem with tying too much of a third movie with Batman Begins, when The Dark Knight has been so much more popular that we'd have to assume many new fans might go to see a Batman 3 without having seen the first.
Another hurdle is that Talia is believed to be in the Justice League movie that at one time was supposed to be already filming by now, but we probably won't see it until 2011 at the earliest. I'm sure the folks at Warner Bros. would prefer not to see Talia in both movies, since whichever one comes out first could ruin the public's opinion of the character.
How I'd do it: A beautiful, exotic, and fabulously wealthy Talia Head (an alias she's used from time to time) appears in Gotham, running in the same social circles as Bruce Wayne. Though Bruce has no idea that she's Ra's al Ghul's daughter, she knows that he is Batman, though she can't prove it or reveal herself as the new leader of the League of Shadows. Still, she wants revenge for Batman stopping her father's plans, so she allies herself with one of Gotham's crazies, providing financing for their schemes and, without revealing Bruce's identity, advice on how to take advantage of his weaknesses.
Casting Guesses: The only name I recall for a casting guess from someone else is Jennifer Connelly. She's a lovely and talented actress, but I'd prefer someone else. Going with the Bond Girl theme, I'd prefer a relative unknown, like how the next James Bond movie has cast Olga Kurylenko. I like the idea of her having an unusual accent, and maybe even being some non-European ethnicity. Theoretically Liam Neeson would be her father, but... I'm not an expert on mixed-race people but I assume it'd be plausible as long as she wasn't really dark-skinned. Some of the most beautiful women in the world are from the middle east, and we almost never see them in Hollywood movies. So I have to assume that somewhere out there is a talented, gorgeous unknown that would be perfect.
Verdict: That's the best I could come up with for a plot idea, and the latter half of it was pretty terrible, but you can see how easily she'd fit into the movies. Although I can think of business-related reasons not to include her, based on the character alone I think she's a great fit in Christopher Nolan's Gotham City.
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.