MapleStory 2, welcome.

Seriously?

Firstly...almost no maps in MapleStory are instanced. Instancing happened very late in the game. Before about two years ago, you had to wait for any party-exclusive maps, like Zakum, Horntail, even PQs. Who remembers the masses waiting for a Kerning PQ channel to open up? That's not instanced. Instancing is a thing that happened very recently in MapleStory and the majority of maps still do not do it. To my knowledge, most PQs are not instanced (haven't played in a while but fairly positive this is still true). All regular training maps are not instanced. The vast majority of the game is not instanced.

Secondly, separate? Maps being separate and a game being open-world are not mutually exclusive things. The overall section of the world is vast, and generally pretty open. Yes, you can only go on certain paths...but how is that any different than any other game? Eventually you hit an invisible wall, or a giant mountain, and are forced down a specific path. Those are your portals. Are you going to tell me a game like Guild Wars is not open world because there are area portals, even though you can literally go anywhere you want in the entire game world(baring some specific story requirements that are generally few and far between)? What about Ragnarok Online? You going to tell me that game isn't open-world because its maps are separate?

I have the distinct feeling that you're using the wrong words for what you really expect/want.