The developer negotiating to buy the former General Motors assembly plant in Doraville, Georgia, is floating the idea of building a stadium for the Falcons there. The Sembler Co. of St. Petersburg, Fla., has proposed using 90 acres for the stadium, with the remaining 75 acres going to mixed-use development, according to city documents. The stadium could have a retractable roof and cost approximately $1 billion. ?It is a possibility, but we don?t know yet what it will be,? Doraville Mayor Ray Jenkins said Wednesday. ?It?s an excellent spot for anything.? The Falcons play at the 16-year-old Georgia Dome downtown, which is older than 20 other NFL stadiums. The NFL frequently awards Super Bowls to cities with newer stadiums. Falcons' owner Arthur Blank has said he wants a new stadium when the bonds sold to build the Dome are paid off, probably around 2015 to 2020. The team?s lease of the state-owned facility expires in 2020 or when the bonds are paid off, McKay said.