In an interview with Game Informer, Shigeru Miyamoto has revealed that Monolith Soft (the guys behind the recent Xenoblade games) are helping put together Breath of the Wild. This makes sense seeing as how Nintendo are not familiar with open world games.

Yes they are involved in this Zelda. People from Tokyo and Kyoto are working together on this. There is a team of over 100 helping work on this project, and their work has really been helpful.