MapleStory 2, welcome.

Of course. I'm sure most would agree that MS is nothing like GTA's sandbox world. But on the other hand, I feel that Maplestory conceptually is more similar to an open world game, with its emphasis on exploration, the possibility of anyone and everyone entering maps, and the non-linearity of game progression. It might not fit the definition of a traditional, open world game, but I can see why myself and others wouldn't label it the opposite.

GTA isn't Sandbox. That's a whole different thing. As for exploration, that is bullshit. Nexon made maps smaller or removed them. Whatever semblance of exploration this game used to have is slowly being ripped away.