I don't think it would be as cool or work as well as everyone thinks it would
When you open up a game like that you lose a LOT of narrative focus and that's where these games shine
People saying to bring in the batmobile, I think that would be horrible.
Here is how I see it working:
First don't think of a huge open world like GTA V, but more like Sleeping Dogs which has many areas within it's land mass that are totally inaccessible to the player. Populated public areas should also be inaccessible, unless some scripted event/setpiece alters this condition. If you glide down into such an area, the game would simply turn you back as in Origins and AC when you go to the edge of the play area. The game would still be set at night, so outside of certain central areas there would still be very few people around and very little traffic on the roads. Certain areas could become busier at certain times, perhaps a large parade takes place at some point during the story.
The Batmobile should be the only vehicle Batman can drive (no GTA style car jacking lol). Batman does not kill, so there will be no using it as a weapon or crashing into pedestrians. They could always jump out of the way just in time like in Driver: SF, or the player might simply have limited control of the vehicle outside of specific instances. The Batmobile could be driven simply for getting around the map or optionally as a fast travel system. Specific missions would be designed around it's use (timed races to locations, race through exploding environment, etc), but otherwise it would be quite limited, and just a cool thing to have in the game for fans.
Combat, stealth, and traversal could all be handled in much the same way as the previous games. The big difference would be the variety and scope that the larger populated environment allows for in terms of mission design, in particular sidequests. A civilian population could add another element to the game - more hostages, more crime variety, Batman has to protect/rescue them as much as he has to neutralize the criminals. It also provides for different storytelling opportunities that the largely deserted open worlds of the previous games do not. For example, terrorists might take over a charity event Bruce Wayne is hosting, and the player must slip away, suit up, free the hostages... and fight Maxie Zeus or whoever.
It doesn't have to be a massive departure, the open world environment just needs to be really well designed. I apologize if this entire post comes off like the rantings of a madman.