That project, one of 10 Chicago-area locations proposed for Amazon’s planned second headquarters, could cost up to $10 billion to develop. Sterling Bay has unveiled plans to build a 20,000-seat stadium with a retractable roof on the site for a United Soccer League team and other sporting events, as well as concerts and other entertainment. Sterling Bay Managing Principal Andy Gloor declined to comment. An Equity Commonwealth spokeswoman did not immediately respond to requests for comment.