The Herald have recently done a series of articles about Scotland's Railways and published an article by rail engineer and YouTuber Gareth Dennis with his view on where new rail lines in Scotland could/should be in the future.

www.heraldscotland.com
He also did a video going into more detail on these proposals:
Certainly they are ambitious proposals, however I'd be surprised to see any of them built (certainly in the next 25 years) with perhaps the exceptions of possibly a line out to Fraserburgh and an extension of the Borders Railway to Hawick.


Reimagining Scotland’s railways from the Highlands to the Scottish Borders
Rail expert Gareth Dennis reimagines the future of Scottish railways. Here is how it could look from the Highlands to the Scottish Borders.
Scotland’s railway network – all 2730 kilometres of it – is one of dramatic contrasts. Glasgow’s railway is the densest urban system outside of London, while the West Highland Line is easily the most spectacular railway in the UK and is one of the most scenic in western Europe.
In the here and now it has its challenges, but what potential does this system hold? If we look to the second half of this century, how might Scotland’s railway look different to today, and what opportunity for the country might this unleash?
He also did a video going into more detail on these proposals:
Certainly they are ambitious proposals, however I'd be surprised to see any of them built (certainly in the next 25 years) with perhaps the exceptions of possibly a line out to Fraserburgh and an extension of the Borders Railway to Hawick.