People need to stop talking about a Subway extension. It just doesn't stack up economically to build a modern metro, the population isn't dense enough so you can deliver the exact same benefit with a tram. Plus a circle by a circle is a bit shite for most journeys, they wouldn't be compatible as the new one will be bigger gauge. Just leave the subway as it is now , extension chat is pointless electioneering distraction or a slow news day at the evening times.
Focus on a tram from Paisley - Airport - Renfrew - Braehaed - QUEH - Govan - City centre - out east overland or through the London Road tunnel making it useful for Celtic Park. This has been muted recently and should be the first step in a large tram network for the city, new lines and converting the Maryhill and Cathcart Circle heavy rail.
It solves the airport problem and provides useful light rail connections to areas only served by bus. The airport link is a shite idea as it only goes from Central, people aren't going to cart all their shit into town and then onto another train or worse the subway or bus. Anyway the platforms and the new track have all been taken up by vastly improving Ayrshire services which delivers far more benefit.
Trams are essential for credible cities. Yes the Edinburgh one went tits up but once it gets to Newhaven everyone will love it. Most European cities have them, poorer and smaller cities than Glasgow and Edinburgh. Fucking Montpellier had 4 extensive tram lines and no cars in the city centre (all the roads in disappear underground into car parks) when I was there 10 years ago, a small provincial city of a couple hundred thousand.
Long term a tunnel connecting Central and Queen St so you can have through trains and get rid of all the capacity issues at Central. Billions for this though so not happening as part of UK.