The National Building Museum has an exhibit now of plans for buildings in Washington that were never built, and the Post has a slide show. I love thinking about how different the places I know might have been. Above John Russel Pope's plan for Lincoln Memorial.
Design for the President's House submitted by "AZ," probably a pseudonym for Thomas Jefferson.
Alexander R. Esty's design for the Library of Congress, ca. 1880.
Design for National Galleries of History and Art by Franklin Webster Smith, 1900. The project would have stretched from 17th Street, near the White House, to the Potomac River.
Plan for Paul Pelz for a new Executive Mansion on the site of Meridian Park.