NATICK — Developer Stuart Rothman sent a clear message to the Planning Board on Wednesday night: Work with him on his latest development proposal at the site of a downtown business block destroyed by a raging fire, or he will likely walk away and keep the property in its current vacant state. Rothman owns the block at 1 South Main St., where an eight-alarm fire destroyed several businesses on July 22, 2019. His latest proposal includes what he called a “Natick pop-up hamlet” that includes 10 separate spaces — each 500 square feet — that are built off-site, trucked in and joined together, and rented to small retailers and artists. An interior courtyard could potentially serve as space for kiosks and a venue for summer music performances.