I think the term "open world" is mostly meaningless these days. Skyrim, for example, is called an open world game. But... It's not? At least not by the definition that "open world" originally meant, which literally was just a continuous game world with no loading screens between areas.

Now it just kinda means "game with big outside map." Unless I can walk into a building without seeing a load screen, I don't consider it to be truly open world.

Dark Souls is a true open world game, even though it's not big or has vast open fields, while, again, Skyrim is not because going into a cave, or a house, or even a major city, requires loading a new level, breaking up the world.