They should obviously try to get back the JNH/Zimmer team for the sequel and just stress that they need a darker atmosphere for the film as well as a few memorable themes that aren't just hypnotic drum/thunder beats.
If they can't get back the JNH/Zimmer team or find that they want a different approach to the score/music of the film... Well, here are a few people I wouldn't mind seeing get a chance to put together a score for The Dark Knight:
-David Julyan (Did great work on Nolan's first three released films Following, Memento, and Insomnia and also reteamed with Nolan on 'The Prestige'. He'd be a great fit for the movie if it does, infact, take a darker and more desperate shift in tone.)
-Howard Shore
-Craig Armstrong
-James Newton Howard (I think that if he were to score the film alone, Howard would be an excellent choice as well. His team up with Zimmer did produce a good score, but I have the feeling that if he were pushed to go it alone... We'd find a Batman movie with a darker score and a few new thematic pieces.)
-Patrick Doyle (It'd be stupid and cliche to say that since Nolan and Doyle are both brits, they should know each other and be in the same realm of working, so I won't say that anymore than I have. However, Doyle did haunting dramatic work on Warner's epic fourth installment of the Harry Potter franchise 'The Goblet of Fire'. His style isn't vastly dissimilar from the style that Zimmer and Howard effected in the first film, so there wouldn't be a major shift in tone musically for the sequel. He's also just damn good and deserves to do another epic flick... besides the impending flop that is Eragon.)
-Harry Gregson-Williams
-John Williams (Might be a bit past his prime, but still puts out damn fine work and wouldn't another DC superhero theme by John Williams be a thought you wouldn't want to surrender? I didn't think so. Check out 'Catch Me If You Can' and a few of the recent Star Wars films to see evidence that he's not only still got it... but still has the uncanny ability to put together delightfully memorable music for film.. and judging from those Star Wars prequels and War of the Worlds, he can also go dark too.)
-Nathan Johnson (If you haven't heard of this guy, don't fret, he's still on the way up. He composed the wonderfully bleak and haunting score for the recent indie flick 'Brick' by Rian Johnson, and it turned out to be one of the most enchanting examples of film scores that I've witnessed in quite some time. He'd be a great, if unproven, addition to Nolan's crew on the Batman films.)
That's it. They're probably get Zimmer/JNH back, get one of them seperatley, or just use Nolan's main man Julyan (which if you've read the whole post, you know I don't mind a bit)... But it's nice to list a few favorites of your own.