As requested by a commenter a while back but I've been slow to get to, Vicki Vale is a recurring love interested from Silver Age Batman stories (which mostly read like the Adam West TV series, with some wacky science fiction mixed in). A reporter for the Gotham Gazette, Vale has a lot in common with Superman's Lois Lane. While dating Bruce Wayne and reporting on the activity of Batman, she often has suspicions that the two are one and the same, but by the end of the issue Batman has managed to fool her into thinking her theory can't be true.
Vicki Vale basically disappeared from the comics in the mid-1960s. However, in the 1989 Batman movie with Michael Keaton, Vicki Vale was written in as Batman/Bruce's main love interest, played by Kim Basinger. This resulted in a brief return to the comics, now as a TV talk show host.
Pros: It's never a bad idea to put a gorgeous redhead in a movie (there's a story -- I don't know if it's true -- that the original Vicki Vale was drawn with a then-unknown Marilyn Monroe in mind, which you can read about here). And as with every other female character I've mentioned, Batman will probably need a love interest of some sort for the next movie.
Cons: Most people associate Vicki Vale with the Tim Burton/Michael Keaton Batman movie, and the Christopher Nolan/Christian Bale movies seem to intentionally distance themselves from that franchise.
How I'd do it: We left off in The Dark Knight with Batman as public enemy #1, so in addition to the cops going after the Bat, the local media would be hot on his trail. Enter Vicki Vale, tough young reporter looking to make her name by breaking the Batman story wide open. She seems to be making more progress than anyone else, so to keep a closer eye on her, Bruce Wayne asks her out. Still heartbroken over the loss of Rachel Dawes, he thinks he won't get emotionally involved, but of course he does.
Casting Guesses: It'd be from the same pool of actresses as Poison Ivy. But while I'd insist on Christina Hendricks for Miss Isley, I think Amy Adams is the best choice for Vicki Vale.
Verdict: I think the connection to the earlier Batman movie makes Vicki Vale somewhat unlikely, but she's a decent enough character who doesn't need any tweaking to fit this universe.
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.