Having recently read The Devil In The White City, I've become enamored with World Fairs. My current TL, for example, includes a proto-Fascist state showing its colors through the Exposition Universelle 1889, which isn't just a throw-away gimmick, as you'll see. (The New York Columbian Exposition of 1893 will be, I'll admit)
So, if you've had ideas for alt-World's Fairs, please, post them here!