Only a single Central Scotland Airport with both EDI and GLA becoming shopping malls and PIK redeveloped as an aviation engineering hub of some sort, would give us the numbers to make a real impact but it sadly isn't going to happen.
When I started work at GLA in 1967 there were moves by several people in the industry to promote the Central Scotland Airport idea (if I remember correctly Mike Bishop was one of them) but it was Glasgow Corporation, who had just invested heavily in building the new airport at Abbotsinch, who unsurprisingly scuppered it. Edinburgh, or more correctly Turnhouse as it was at the time, was owned by the Ministry of Civil Aviation who had invested zilch in it and would probably have been happy to see it go.
We need to face the fact that we don't have enough passenger and cargo throughput to support two major and one minor airport in the central belt and so the nearest we are going to get to a proper hub with global scheduled services is a developed Edinburgh. I say that simply because of its geographic location between Glasgow and Edinburgh (not on the wrong side of one of them), nearer to Dundee and the Stirling/Falkirk conurbation with excellent motorways linking it with all of these places, and its proximity to rail infrastructure (when the Cogar interchange opens) which will allow through train services to get there from all parts of the country as well as the North of England. I am afraid that Prestwick is doomed as a commercial airport of any significance but I do see Glasgow living on with limited scheduled services on busy, mainly domestic, routes, but predominently as the country's bucket and spade holiday charter airport as well as it picking up most of PIK's cargo business.
Now off to don hard hat and avoid incoming flak from west of Ballieston.....
Link to an interesting paper on the subject from the David Hume Institute.
http://homepages.ed.ac.uk/mainbg/Files/csa%20study.pdf