Bob Chapek announced today at D23 Expo 2017 that a new venue for world-class entertainment is coming to Main Street, U.S.A. at the Magic Kingdom.
This new entertainment venue will be located adjacent to Main Street, U.S.A, and the design is based on the iconic Willis Wood theater in 1920s Kansas City, where Walt lived after he left Marceline.
“Across Walt Disney Parks and Resorts, we’ve been upping the game in Entertainment with shows like Frozen Live at the Hyperion,” Bob Chapek, Chairman of Walt Disney Parks & Resorts said. “It’s been a huge hit at the Disneyland Resort, and I’m sure many of you have seen it. This new theater at Walt Disney World continues that streak by bringing a new venue for world-class entertainment to the world’s biggest park.”
Although the Magic Kingdom is definitely not the world’s biggest park, as Chapek claims, a new theater akin to they Hyperion Theater at Disney California Adventure will be a welcome addition to the most-visited theme park in the world.

No dates have been released as of yet.
I think they mean biggest as in most popular, biggest attendance.