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.
Exactly. Make it infrastructure that's hidden away from the front end. Find some way to wrap up duplicate groups into larger categories or something, and figure out a way to migrate accounts if your home instance tanks. That would cover all my concerns.