RockSteady didn't set out to make an open-world game with Batman: Arkham City

“When we started the game we didn’t sit down and say, ‘Let’s make an open world game’. We looked at what we be a fun game to make and a fun game to play. That’s what drove the open world decision. It’s more about what we think is a good fit, what will be fun and what gets the team inspired, rather than picking a genre for the sake of it. It was an awful lot of work – no doubt about that – there was a lot of late nights and a lot of weekend working, but we think it was all worth it. So that would never put us off a similar experience in the future.” - Rocksteady game director Sefton Hill

I can't even imagine the amount of work that went into the game. Even worse would be thinking about revisiting it a year later for the Wii U version!

Link