The brick and cobblestone walkways in the Washington Square plaza would be replaced with more pedestrian-friendly concrete, a performance space created and a permanent Christmas tree planted under a redesign proposal city leaders are considering. The Washington Street Shoe Historic District Commission took a first look at a concept for the redesign of the plaza opposite the Haverhill post office at its regular meeting Wednesday. Lisa Decker, the AECOM landscape architect on the project, suggested bricks could be used as a decorative element on the edges of the walkways to reflect the many historic brick buildings on Washington Street. Commissioner Paul Bergman asked if traffic-calming elements had been considered including narrowing the width of Washington Street inside the square and making the park area larger to force drivers to slow down. He also said city officials instructed him to preserve the five parking spaces that run the length of the plaza on Washington Street.