To summarise some of the thoughts in previous posts, might a revised timetable look something like:
Highlander, 2 portions ex Euston, one to Inverness, and the other to Fort William via Glasgow Queen Street Low Level.
Lowlander, 2 portions ex Euston, whole train to Edinburgh, one portion forward to Aberdeen via Fife.
I'm tempted to play a tune or two on this... how about:
The Highlander runs to Glasgow Central instead of Edinburgh, and performs the split there. It could also detach/attach a couple of sleepers there, to give more sociable boarding / alighting times.
The Lowlander would have no need to run via Carstairs, so runs direct to Edinburgh arr circa 06:45. After dividing and loco change the Fife / Aberdeen portion could depart about 07:05, to arrive in Aberdeen about 09:45.
There are further tunes to be played when you take into account electrification through to Stirling.
I quite like the sound of this one. The Highlander not going via Edinburgh would of course take away the up direction only service Falkirk acquired by accident of geography when the Highlander was concentrated on Edinburgh - traditionally Falkirk was not on the Sleeper network (other than for the internal Edinburgh to Inverness), probably less of a loss than some of the places I list below which used to have a sleeper service.
The FW portion, having detached in Glasgow Central could take the most direct route via the curve at Rutherglen and the Argyle Line.
Another option might be for the same 'portion' to serve both Edinburgh and Glasgow, as servicing is at Polmadie, Edinburgh first then forward at 07.xx to Glasgow albeit with the potential problem of finding a path in the morning peak. Leaving stock in Edinburgh for later de-training would just create an additional ECS and associated resources.
I think part of the problem the current operator has is they inherited a 'one size fits all' service structure which has been in place for at least 25 years and when set up was trying to serve most (but not all) of the markets served my the more traditional sleeper network. Markets have clearly changed and a re-evaluation would appear prudent.
Whilst the Lowlander clearly targeted the Glasgow and Edinburgh 'cities' markets (traditionally the largest markets), at that time the function of the Highlander would have seen Aberdeen and Inverness as the most important markets to serve but at the expense of very early arrival times at Perth, Dundee and points south, some of which previously would have had a decent level of sleeper usage.
Aberdeen for example received three trains from Kings Cross the first arriving just before 07.00 clearly aimed at the Aberdeen market, the second arriving about an hour and a half later, just about tolerable for Dundee but OK for the stations northwards, and the third which only left Edinburgh around 07.00 (as a portion off a Kings X Edinburgh), ideal for Fife and Dundee. And of course from London these departures were spread over a three hour plus spread window, thus suiting many different needs.
Similarly Stirling and Perth were served by the Euston to Perth, much later than the Inverness service which, certainly northbound, as now was not attractively timed for Perth.
Other flows of course had been abandoned, and concentrating only around the border and northwards, not just Stranraer, but also such as Dunbar and Berwick, and Dumfries and Kilmarnock. Whilst some may doubt the volume for the latter two it must have been reasonably significant back in the day as one of the two sleeping car only Glasgow services took this route (and also took the Carlisle sleeping cars portion, much more civil than today's offering) as well as the mixed seats and sleepers Glasgow train. And of course the West Country (and for some years Poole) also had a Scottish sleeper although until the 70's that had not gone south of Birmingham.