I think I'd like to see Batman doing a little more detective work. I read the novelization of "Dark Knight". It's not particularly interesting - it's mostly retelling everything you already saw on the screen. I had hoped for some deeper insights into the individual characters, but it's really just the screenplay made into a book. But there have been some extra scenes, and the ones I liked where those describing Bruce Waynes investigations.
I am not sure how well it will fit into the established setting, but surely the Nolan-Batman does some legwork (The mayor-assassination and the Lamborghini-scene are that in some way). I suppose the style could either be more "Film Noire" or maybe like a thriller á la Seven. Someone like the Riddler and Catwoman can make sense in such scenarios.
Of course, I suppose you still need something to support a lot of action scenes, so maybe it will require a mix of two villains operating differently or on different scales.
In the Dark Knight, Batman first seems to ignore the Joker because he is focusing on the "Big Goal", but eventually it turns out that the Joker is critical to this. The next movie might have to force Batman to decide where to focus his resources and efforts - a mysterious serial killer (maybe more then just individuals - he might sometimes blow up buses and buildings), or a crime syndicate trying to to retake Gotham City, instigating gang wars and cop shoot-outs.
