MapleStory

Grinding, grinding, grinding. The staple of a true Korean RPG, now in multiplayer form! Many secrets are to be found in what is perhaps the most successful free to play game in the world.

Mushroom Shrine
These NPCs exist in the data of GMS, and are partially functional, but have yet to make an appearance outside of the Japanese version where they originated and are now unused there as well.

Beta Free Markets
During beta testing, several towns had dedicated free market entrances and rooms for players to trade in, themed after the town they were located in. When the game was officially released, the free market was removed while it underwent a large revamp, with a universal free market entrance eventually appearing in most towns to replace the individual ones. While some of the maps were modified and recycled into the current free market rooms, the original entrance maps were cut and are now completely absent in the game's client, save for string data referencing them. Interestingly enough, a few of these maps were cut before players even got to see them, due to some areas not being officially released at the time, despite being fully functional in the beta client.

Coke Town
In the Korean servers, there was an event area dedicated to the Coca-Cola® company, likely under sponsorship, in an event called CokePLAY. It was initially only accessible to people who used the cash shop, but was eventually made available to all players before it was removed. It also appeared in JMS for a short amount of time under a promotional campaign. The town never has and likely never will appear in GMS, but despite this parts of it exist in the data, most notably in monsters and NPCs. There is no map data whatsoever in GMS aside from a background effect and strings referencing the map titles, which are fully localized.