We now know their will officially be a MapleStory sequel. Nexon released a MapleStory 2 teaser site in Korea this week. The site bears no actual information aside from the secret teaser trailer above, but we do know MapleStory 2 is set before the events in MapleStory and that it will sport 3D graphics in an isometric view.
A FAQ posted on the Maplestory site explains that neither game will be interconnected, but will share some monsters and areas. The Current MapleStory devs will continue to produce new content for the original, while MapleStory 2 is developed by N Square, who also happen to be developing the sequel to Mabinogi titled, Mabinogi II: Arena.
MapleStory 2 is scheduled to enter closed beta testing starting next year. No information regarding a western release has been given. For comparison, the original MapleStory was released in North America two years after its Korean debut.