Would have been the site of numerous new buildings suitable for government use; the proposal above does not feature a grand promenade, and does not appear to widen East Capitol Street itself. Instead it would have widened Constitution and Independence Avenues.
Alternatively, it could have been named the "East Capitol Campus".
There were different versions of this basic proposal, with a later version including a depressed north-south roadway crossing beneath Lincoln Park just east of 11th Street NE/SE. (The 1955 Inner Loop Freeway study would here route the I-295 East Leg, via a right of way alongside and not in tunnel beneath 11th Street NE/SE).
The East Capitol "Mall" would have displaced hundreds of late 1800s era townhouse dwellings (generally of the more architecturally desirable types), and for that reason was stopped. Today, virtually all of these dwellings remain extant, viable and vibrant, truly justifying the cancellation.
http://eye.jimbomania.com/pix_cherryblossoms.html (David Eyerman)
The 2005 annual conference of the D.C. Historical Society included a presentation on this general proposal.
An amazing link: