To celebrate its 75th anniversary as a public institution, the Walker Art Center will embark on a $75 million renovation of its rolling campus, the final stage of an expansion that began more than a decade ago with the addition of a new building by the architects Jacques Herzog and Pierre de Meuron. The new project will restructure the museum’s grounds and add a highly visible entrance pavilion, and it will also tie the Walker more closely to the Minneapolis Sculpture Garden across the street, home to “Spoonbridge and Cherry,” the sculpture by Claes Oldenburg and Coosje van Bruggen that has become a Minneapolis landmark.