Ok, the way I see it, AA was based loosely on the comic storyline of the same name, right? I would say they are looking at the Batman universe for inspiration for stories. Sure, the Titan thing was new, but Bats trapped in Arkham was from the comics.
So, I've always thought No Man's Land would be a brilliant gameplay environment. But it's such a big story and setting that it would be near impossible for a first game. Then I played AA and I could tell two things. One, these guys could build a No Man's Land game out of this. They handled an open environment, great story, and paced the game well. The nearly perfected the combat and the sneaking and the bat-gadgets. Two, these guys were experimenting. I got the same feeling in AA I got in GTA 3. It felt like there was more to this, but the developers restrained themselves, they were testing their skills and the market, the consumers. I could tell that no matter the setting, the next Batman game would be the one they really wanted to make. And remember how much Vice City blew us away.
So, why No Man's Land? First, you can use all the villains and hero cameo's you want. You have a desperate situation that looks near impossible, a city beyond the brink, and that is when Batman is best. Recall the feelings in Batman Begins when Arkham is broken open, the fear toxin is spreading, and Ra's is about to destroy everything. Intense, and impossible, except or Batman. No Man's Land is brilliant because Gotham is already lost, totally and completely. I can think of no more compelling Batman setting. Also, with a fragmented city, separated by the different gangs (cannon fodder enemies) and major players (more important battles and bosses), you can still slowly integrate the open-world setting without fake limitations set on players.
The biggest thing missing from AA for me was anyone in the Bat Family besides Oracle (love!). NML included EVERYONE! Tim Drake as Robin, Nightwing, Oracle, Huntress, Azrael, a new Batgirl. This means cameos in story mode and great Batman dialog ("In twenty-four hours, Nightwing will be inside Blackgate Prison. In forty-eight hours, it will be under his control" and "You have one hour to get across town. If you can't do this, you don't deserve to be on my team.") and I would hope, and this is very likely, a co-op mode. Imagine a cross between the Challenges in AA and the Spec Ops in CoD MW2.
Two major changes would need to happen, however. NML had a great arc and ending, but the two ending elements (The person(s) behind NML and the Joker) came out of nowhere, almost. It was kind of like they decided to make a cool setting and determine an ending later - which they did, but that's ok in comics. In a video game, integrate your major players early on.