Kaplan joined the World of Warcraft team, initially working on quest design and world building. His understanding of narrative and his experience with online games proved valuable in creating engaging and immersive content for players.

Kaplan quickly rose through the ranks, eventually becoming the lead designer for World of Warcraft. In this role, he had significant influence over the game's direction, overseeing the design of new content, classes, and features. His leadership helped shape the game into the global phenomenon it became.

As lead designer, Kaplan played a crucial role in developing World of Warcraft's first two major expansions: The Burning Crusade and Wrath of the Lich King. He helped craft the storylines, create the new zones, and balance the gameplay, contributing significantly to the success of these expansions.