Let's see here, what could they do to bring some youthful energy to the dark and gritty Dark Knight...
-Dick Grayson
-Tim Drake
-Barbara Gordon (have they even mentioned her in the Nolan films, and I mean her as Barbara Gordon, not just Batgirl?)
-Bat-Mite (if they wanted to be stupid and pander to kids, despite how Bat-Mite is pretty enjoyable, if a little annoying, in Brave and the Bold)
-Ace the Bat-Hound
Who they'd probably bring in to keep the Dark Knight, well, Dark:
-Jason Todd (second Robin, killed by Joker, generally unliked by Batman fans for jerkass attitude)
-Cassandra Cain (as Batgirl, do a google search)
Also, it's already a forgone conclusion that they're going to bring in The Riddler as the movie's antagonist, but my question is who's next? To me, the dark, grim and gritty feel of The Dark Knight screams primarily of three villains that would thrive in its environment: Mr. Freeze, Scarecrow and Bane. However, they'd really have to go into depth with Freeze and Bane's backstories and it wouldn't hurt to flesh out Scarecrow more into a full-fledged supervillain, especially since he's been pretty much shafted by the Batman movies (ignored by the Burton and Shumaker films, glanced over in Batman Begins and appeared for about five minuets in Dark Knight). To me, here's how you implement Freeze, Bane and Scarecrow properly into the Nolan Batman films.
Mr. Freeze: Keep the backstory (cryogenecist who freezes his wife until he can find a cure for her (currently) incurable ailment, gets shoved into chemicals by dickhead boss and goes on a revenge-fueled spree), make the cryo-suit and freeze gun a little more sleek and compact, maybe downplay the wife thing a little bit. Or to put it quicker, "go watch Batman The Animated Series episode "Heart of Ice" and take close notes.
Bane: Okay, you're gonna have to go into backstory detail for Bane if he's in the movies as the villain. I'm serious, Bane is a mess to do as a Batman movie villain. He was born to serve his father's prison sentence in Peña Duro prison, he pretty much strong-arms his to the top of the prison's tough-guy list, is forced to become a test-subject for Venom, escapes Peña Duro and goes after Batman because he'd been seeing demonic bats in his nightmares all his life and was convinced that Batman was the source and seeks to conquer it. Best-case scenario, you end up with a modified Knightfall, where Bane breaks Batman's back and (nearly) cripples him (I say "nearly" because in the comics he DID cripple Bruce Wayne, but in the movies that'd probably not be a smart move), worst case scenario you end up with a generic super-strong mook (see "Batman and Robin"). Also, Bane is a tactical mastermind and not just a druggie version of the Hulk. He's polylingual (speaks multiple languages, among which are Spanish, French, German, Russian, Mandarin, English, Urdu, Farsi, and Latin), a math genius (despite having no actual mathematical education, he read countless encyclopedias and books in prison and is knowledgeable in Mathematics, various Sciences, History, Geography, Medicine, and most likely more) and possesses Olympic-level strength and reflexes, and that's just when he's not on Venom. On the stuff, he can lift up to 35 tons (about 35 Ford Mustangs, to put that in perspective), and he's still all the stuff I mentioned before. There's a reason this guy is the only person who's claimed to have "broken the bat" and it's because he's just awesome. And as one final thing, he has to have a spanish accent. Bane IS latin american, like it or not. Sure, not an easy thing to find the right image for the guy, but just use CGI for when he uses the Venom.
Scarecrow: We've already seen Scarecrow as an established bad guy in the Dark Knight, just don't make him a one-note deal. Scarecrow's not just the fear gas, he's a crazy psychologist who has a fascination with fear to the point that the only way he can actually feel fear is to fight Batman (or wear a Yellow Lantern ring, but that's another crisis crossover [http://dc.wikia.com/wiki/Blackest_Night]). Look, the point is Scarecrow has been majorly downgraded from one of Batman's deadliest foes to quasi-drug dealer. Sure, he had that one moment in Batman Begins, but if a taser is what it takes to take him out and NOT Batman, something is wrong. Bottom line, promote Scarecrow to important villain.