The costs involved in building a new national stadium means it will never happen unless the government or lottery subsidise it heavily. Only other way it could happen is a combined Rangers/ Celtic or SFA / SRU stadium. SFA building a new stadium for half a dozen Scotland games and 6 cup finals / semi finals a year, which wouldn't all be sellouts, is not likely or affordable.
If it was ever built would almost certainly need to built in Glasgow or Edinburgh. Probably the best option is to strike a deal with SRU to use Murrayfield for say 4 big Scotland matches a year and the Scottish / League cup finals. All semi finals and smaller Scotland games could be played at various other football stadiums.