There was a flicker of hope earlier this month, that the FBI would do its part to make Pennsylvania Avenue the lively and impressive "Main Street of the Nation" it ought to be.

But the flicker died.

The FBI is again as silent and inscrutable as the super block-long concrete wall along the facade of the J. Edgar Hoover Building that divides the eastern and western stretches of the avenue with an intimidating dead space.

The flicker of hope was that FBI Director William H. Webster promised to at least consider piercing that forbidding wall with small shops and to invite some human activity into the forecourt of the building.

Webster wrote Sen. Robert T. Stafford (R-Vt.) that he had ordered an assessment of the security problems involved in making a small part of this public building public.

Since that letter was written three weeks ago, "there is no further news," says the FBI press office.

It was a bad idea in the first place to make the headquarters of the nation's secret police the most dominant building on the nation's foremost ceremonial avenue. But J. Edgar Hoover wanted a prominent monument, and in those days - early in the '60s - J. Edgar got what he wanted.

The incongruity of placing a high security institution in the heart of a tourist and shopping center should have become apparent when the architects of the Hoover Building tried to make it smile a little. They proposed an arcade along the Pennsylvania Avenue facade.

Mr. Hoover was horrified. He allowed that muggers might hide in those shadows and molest his employes.

So in place of their arcade the architects tried to modulate the Pennsylvania Avenue facade a little. This, however, was frustrated by a then-member of the Fine Arts Commission, Gordon Bunshaft of Skidmore, Owings and Merrill, the architect who designed the Hirshhorn Museum and Sculpture Garden. Wielding a ballpoint pen on the architects' rendering, the formidable Bunshaft sketched in that formidable wall.

Times and formidabilities have changed, and Bunshaft's retired boss, Nathaniel A. Owings, is now quoted as saying "It's always been our belief, intent and desire" that retail shops should exist in front of the FBI building.

What is confusing about that is that Owings was chairman of the President's Advisory Commission on Pennsylvania Avenue, as well as the head of Bunshaft's firm, at the time the final design of the FBI building, blank wall and all, was approved.

But the politics of architecture is as confusing as the architecture of politics.

At any event, it was not Owings, nor the present Pennsylvania Development Corp., but the city's Municipal Planning Office under Ben. W. Gilbert, which in January 1976 officially proposed that retail space be carved out of the FBI colossus. The idea became feasible with the passage of the Public Buildings Cooperative Use Act, which permits the federal government to lease space for shops and other public amenities in its buildings.

The city's proposal for an "FBI Plaza" with shops and kiosks, worked out by urban designer John Fondersmith, argues that if the dead space between Ninth and 10th Streets is bad now, it will be even more disastrous in the future.

It will break a flow of activity generated by new officers and retail complexes west of the FBI building, and shops, exhibits and housing east of it. New development along Pennsylvania Avenue is no longer a pipe dream.It is underway.

Fondersmith showed in some detail how the 11 bays of the building fronting Pennsylvania Avenue could be opened up by simply removing the concrete panels. Three bays would remain open as a gate into the FBI building's forecourt, and the remainder would house shops measuring about 30 by 30 feet, offering books, souvenirs, refreshments, film and other-items of interest to tourists, including those who visit the FBI displays on crime fighting and espionage.

The MPO's study was submitted to the FBI, but Webster's predecessor. Clarence M. Kelly, rejected the idea on the ground that foreign spies might infiltrate the ranks of shopkeepers and customers.

The bars and "adult" bookstores and peep shows that surround the J. Edgar Hoover Building now are presumably so sleazy that no self-respecting foreign agent would go near them.

Left to its own devices, the FBI will probably take its own sweet time to "assess the security problems," that is, figure out how to keep its restricted areas restricted. Once the Bureau has decided where to post its guards and place its locks, it will consult with the General Services Administrator's Public Building Service, which is actually in charge of the building. GSA estimates that some 40,000 square feet of commercial space can and should be made available.

GSA also estimates that at the speed at which this matter is moving, it will be at least five years before an appropriations request for the necessary alterations of the Hoover Building could pass Congress.

The rest of Pennsylvania Avenue, however, is coming along at such a splendid pace that it is time for the attorney general and perhaps even the president to nudge the FBI out of its paranoia.

A few more shops in downtown Washington may not be of earth-shaking importance. But speedy action on the MPO proposal would be a symbol that the FBI is in earnest about a less arrogant and more human face. CAPTION: Illustration, Pennsylvania Avenue front of FBI Plaza proposed by the Municipal Planning Office