It will be interesting to see what comes from the ministerial announcement in September that the I7C HST fleet is to be replaced.
Modern Railways magazine considers it likely that Transport Scotland and ScotRail will consider acquiring the Class 222 Meridian fleet which is currently operated by East Midlands Railway and is due to be replaced by Hitachi Class 810 bi-mode units.
These trains, built between 2003 and 2005 by Bombardier, are currently in 5- and 7-car sets and replaced HSTs on the Midland Main Line, cutting the journey time between London and Sheffield by 12 mins as these units have better acceleration than the HSTs they replaced. Given the short I7C formations they might not offer such a noticeable improvement over ScotRail's HSTs.
The HST fleet has proved expensive to run, as well as having some reliability issues. With "decarbonisation" still planned for 2035 it would be nearly 60 years old by then, and with rolling stock procurement taking many years and being very expensive an interim solution does seem likely.
As always, Transport Scotland and the Scottish Government do not reveal details of plans, so guesswork remains. Whatever rolling stock is procured it must be at least as comfortable as the I7C MkIII carriages.