A massive redevelopment project surrounding Ball Arena that would transform downtown Denver over the next three decades by adding thousands of new homes, commercial property and open space received approval Wednesday from a key city board. The plan also would guarantee the Denver Nuggets and Colorado Avalanche won’t leave any time soon because the city’s approval hinges on a promise to keep the two pro teams playing at the downtown arena. The proposed development would include up to 6,000 new housing units with 18% of those qualifying as affordable housing, a new city recreation center, retail space, a hotel and a signature park with walking and biking trails on the 70 acres surrounding Ball Arena adjacent to Denver’s downtown. The plan would include at least two multi-story towers, although their exact height and size are yet to be determined. The project is being proposed by Kroenke Sports and Entertainment — the company that owns the Nuggets and Avalanche — and it would largely replace an area that now is mostly parking lots.