The local developer Keystone Group is purchasing the downtown building that last housed a Dunkin Donuts with plans to renovate it to make it more attractive to an upscale eatery. In renderings of what the building might look like, the iconic sign atop the building is absent, though it could stay, according to grandfathered city zoning laws. Plans for the building at the high-profile corner of Washington and Pennsylvania streets include adding a second story in hopes of luring a national restaurant. Retail brokerage Midland Atlantic is shopping the site and has the property under contract with a potential buyer, broker Patrick Boyle told Property Lines for a blog post earlier this month. The purchase could include the adjoining space currently occupied by Cartridge World, which would be the “ideal scenario,” he said. The entire space is 2,800 square feet. Dunkin Donuts closed the location at 48 E. Washington St. in late March when its lease expired. The designer is Axis Architecture + Interiors.