We've added a new page to ExpoMuseum.com about the intersection between world's fairs and Walt Disney. It's a work in progress as we still need to find out more information whether Walt Disney visited 1935 Brussels, 1935 San Diego, 1937 Paris, or 1958 Brussels. At least two or three of those seem likely.
Also, I've heard that both 1984 New Orleans' Seymour D. Fair and 1988 Brisbane's Expo Oz were designed by Disney animators. I haven't been able to confirm either of those.