BOSS BATTLES! If I were to have one complaint, albeit a tiny one, about the last game? there were little to no boss battles!
You shine spotlights on Scarecrow, fight joker goons in place of Harley Quinn, run away from Croc, Bane was just a copy of the regular mutated goons you fight regularly later in the game, throw baterangs at ivy (and fight goons), and Joker... I mean... SUPER-Mutated Joker... you dodge him, and? yes, fight MORE goons.
Game was spic. I dunno if a sandbox world would work. I mean anything is possible if done right. I think the beauty of the 1st game was that it was self contained within it's own world. You were stuck on an island and whatever had to be done was going to GET done then and there.
Problem with sandbox games is that at some point the story has to stop so you can venture out and explore the world... doing so means they'd need to fill that world with enough interesting things to do. Games like Infamous and Prototype got old really fast because there wasn't much to do in between stories. yeah it'd be cool to use the grappling hook up a building and spread your batwings and soar over Gotham, but eventually that would lose it's "wow" factor and become meaningless.
How about a multiplayer where you can be Batman and a friend online could co-op as Batgirl or Robin? make challenges that require BOTH people to participate?
Yes, I agree, I would liked to have seen Batman get more 'hands on' with the major baddies in the game. And yes, a sandbox game could work if done right. It would be awesome to have Batman going to different locals throughout Gotham City, like nightclubs to put the squeeze on mob bosses.
Keep it somewhere confined like Arkham. The game was great for being in a single area, which was polished to near-perfection. Sorry, but going open-world would fuck things up big time.