Denver International Airport’s proposed $1.8 billion terminal deal is poised to launch the city into a different kind of contracting that hands over some control of a publicly owned space to private interests for decades. Get ready: That complex, 34-year contract could be a harbinger of things to come. Mayor Michael Hancock and other city leaders see promise in public-private partnerships, which infuse both private money and management into public projects, as they prepare to build out the National Western Center with large new event venues, expand the convention center and plot big changes to the Denver Performing Arts Complex. But as the City Council prepares for a vote Monday night on DIA’s proposed arrangement with a consortium led by a large Spanish company, some members express misgivings about more such deals.