Scotland actually already has the making of one major International airport in Edinburgh.It is conveniently accessible from ALL parts of Scotland and Northern England.
Edinburgh is not and never has been handy for the west of the country. It's not even conveniently accesible from Edinburgh unless you drive.
The central scotland airport would be accessed by new modern infrastructure from both the west and the east. Edinburgh is acessed by the nightmare that is the M8 from the west and has no railway connection at all. The cost benefit analysis wasn't a good one.
A sensible move would be the closure of both EDI and GLA and their replacement with a built for purpose facility for the nation, plugged into the railway network, north to Dundee, Inverness and Aberdeen, east to Edinburgh and west to Glasgow and Ayrshire. However too many parochial wee men would make sure that could never happen.