Corporate offices might make good housing, malls could be useful for community services. Medical centres, libraries, hackerspaces, community courses (volunteer led), open up skylights in some of the old stores and build greenhouses for community gardens, temporary accommodation, kitchens for homeless people (and other services), market stall spaces and short term storefronts for small businesses so people can have a fair go at selling their stuff without being locked into years-long contracts. So many good ideas in this thread!