Easier level design. I wouldn’t be surprised if 99% of open world games just had their landscape generated and then slightly tweaked afterwards.
You can do open world right, oblivion and to a slightly lesser extent Skyrim. But a huge map with not much in it just makes for tedious travel times, or lots of fast travel loading screens. At that point, you basically have separate levels.
Dark souls 1 was good too, but for a different reason: there’s nothing like opening a gate and going “WTF, how did I get all the way back here?” The way it folds in on itself makes it huge, but also gives it a very compact feeling when it comes to traveling around. I’d put it top 3 level design on my personal list.
Far cry 3 was good too. Mostly because wing suit and helicopter thing. Now that I think of it, there’s a theme here. It seems like verticality (and a way to traverse it) really helps a map feel fun. Far cry 3, BOTW, dark souls, all 3 have these huge altitude variations.
