City leaders have zeroed in on a strategy they hope can break downtown Denver out of what Mayor Mike Johnston has described as the area’s post-COVID “doom loop.” The key to that strategy: the expansion of an obscure special taxing authority that played a key part in downtown’s last big boom. Johnston and other city and business leaders stood in front of the dormant fountains outside Union Station on Thursday morning to announce a plan they say could generate $500 million in public investment in downtown Denver over the coming decade. The approach relies on a strategic funding tool that helped turn Union Station from an all-but-deserted bus terminal into an anchor of downtown Denver’s economic resurgence in the 2010s. Namely, the Johnston administration and its partners are intent on expanding the boundaries of the Denver Downtown Development Authority to cover all of the city’s core, including the long-floundering Central Business District. Once expanded, that entity — created to pay off $400 million in public debt incurred building infrastructure around the station — would collect incremental property taxes from participating businesses and property owners to back bonds that can be used to fund a host of economic development work and projects, officials explained. What kind of work?