There a theoretical case for re-introducing a sleeper service from Scotland to Bristol and South-West England, but many on this forum seem to have dismissed the idea out-of-hand. It is possible to derive a suitable timetable, which could reduce costs by integration with the Lowland sleeper between Glasgow/Edinburgh and Carstairs, and swapping diesel/electric engines during a stop at Crewe. Approximate times could be as follows: Glasgow/Edinburgh dep 2300, Carstairs 2345, Crewe 0245/0315, Bristol 0630, Plymouth 0845, Penzance arr 1100, with the reverse timings Penzance dep 1900 and Glasgow/Edinburgh arr 0700. [That would permit use of the North & West main line via the Marches as a diversionary route between Crewe and Bristol.]
However, it is interesting to note that cross-country sleeper services (not to/from London) were mainly a BR introduction, although Glasgow to Inverness was run pre-1914, and Glasgow to Birmingham/Liverpool/Manchester routes are shown in a late LMS timetable. In general, they were not run by the "Big Four" or their predecessors, although there were overnight through carriages, e.g. from Glasgow to Bristol by a variety of routes. I suspect that the cross-country sleeper services were never profitable, and were introduced as a "service improvement" under the direction of the BTC. The case for any re-introduction would have to be based on a potential reduction in air and car travel on the route, with the "negative costs" of these travel modes factored into any economic equation. Market research would be needed, and changes in the relative costs of different travel modes and accommodation mean that one can't extrapolate from the situation 30 years ago. The situation may change over the coming years as air services from England to Scotland are gradually withdrawn; those from Cardiff to Scotland ceased pre-Covid in the last 12 months.
In reality, I don't envisage the re-introduction of sleeper services, and future acceleration of day trains between London and Glasgow/Edinburgh will threaten the continued existence of a sleeper service on this route.