Four massive mixed-use projects that would ultimately result in billions of dollars of new, skyline-changing development in Dallas took big leaps forward this week. All told, if the ambitious plans come to fruition, the city of Dallas will have about 2,000 new multifamily residential units, hundreds of new single-family homes, more than a million square feet of new office and retail space, multiple hotels, a 38-floor mixed-use tower, and an 80-floor skyscraper that could be the tallest one in town.