Yes I don’t think there’s anywhere in the uk that really needs roof windows!In mountainous areas it's often out and up, but I find for UK purposes (given our smaller mountains) that tapered bodysides as found on e.g. classes 158, 170 and 22x provide well enough for that.
If there was a proposal to life extend 158’s to specifically work scenic lines in years to come then they would be ideal with some proper work done on them. I’d actually like to see the hybrid MTU pack installed on them for a start off to bring them a bit more up to date. Then there‘s the air con (I think you suggested the excellent system in the Chiltern 165’s which has always been spot on when I’ve travelled on them), they could look at the old W&B layout internally with 10 tables per coach rather than 8 and larger luggage stacks but then a solution would be needed for cycle provision as the 158’s are pretty inadequate in this area.
To be fair I think there’s little to no chance of a small amount of ‘scenic trains’ being built or modified much into the future. Most stock used on scenic lines needs to be adequate for other services too. Small fleets of stock for specific routes isn’t particularly cost effective.
