Under a proposed plan in Boston, the Massachusetts Mental Health Center in Mission Hill would be demolished and in its place Brigham and Women's Hospital would build a residential and medical complex. The hospital submitted to the city a proposal for a multibuilding development of more than 600,000 square feet. Brigham and Women's plans include a 57,000-square-foot laboratory and office building that would also include outpatient services, according to documents filed with the Boston Redevelopment Authority.