I can see an Open Access company desperately looking round the map to try to find a gap in the market that isn't directly served already, or is just a copy of two existing services - since they have to ensure that it's not primarily abstractive.
That's why Open Access come up with ideas like Swindon to Birmingham, to try to exploit a niche between existing TOC boundaries.
That's different to the way that any TOC (or the DfT) would do sensible planning though
The Renaissance plan of Blackpool - Glasgow made more sense as a direct service, given the amount of people from Clydeside who go on holiday there or even retire there (with the "Bluenose" bars etc). That's a clear market.
Ah, so Nottingham doesn't need a link to large conurbations in the Midlands, but does need a direct service to Glasgow almost three hundred miles away?
Nottingham has a service to Birmingham, where you can easily connect to those destinations
Well, yes, but that's a fairly vague argument that could be used to justify a direct link between any places. Why is a direct link from Nottingham to Glasgow so desirable?
It's not just those two cities getting a direct link, and they are both large population centres! There are large markets inbetween too. In my opinion, the following are under-served with fast decent quality services:
Glasgow-Leeds/Sheffield/Nottingham (XC, is expensive, slow, crowded and fairly unpleasant, all large markets)
Carlisle-Leeds (one fast service per day, all others are very slow)
Appleby-Carlisle/Leeds (lack of fast trains to either)
Settle-Leeds (few fast trains to Leeds/Shipley, a tourist destination itself and gateway to the Dales so could be a sizeable market)
Skipton (lacks direct trains to many places, I appreciate it's hardly a metropolis but its a tourist destination in itself and it could do a decent tourist trade in the summer as it's the gateway to the Dales)
Shipley- Glasgow, Carlisle, Sheffield, Nottingham- essentially for Bradford passengers, plus Shipley and Ilkley residents (a large market when considered together)
Leeds-Sheffield- poorly served in my opinion and the new franchise still won't improve it enough.
Leeds- Nottingham- as above, but worse.
Some enthusiasts do suggest the GSW. Anyone care to justify this?
A minority do, and as I've already said, I personally think that would ruin any hypothetical Glasgow-Nottingham service.
Please don't confuse the "mainstream" with "what a lot of enthusiasts want"...
I think the 'mainstream' on here, would welcome it. The 'normals' (non-enthusiasts) are unlikely to care, until it came to the price of their journey being reduced, or not having to change trains, or stand in the vestibule of a 220 with a smelly toilet...
Does the WCML have spare capacity? Well, you don't want to use the GSW, so presumably you think that it does?
It does, North of Carlisle. Even more so if coal traffic declined. South of Carlisle, not so much
Part of the reason that the ECML and WCML struggle for capacity is that they serve a lot of busy places en route - rather than Settle/ Appleby.
Do they? North of Preston the WCML barely serves anyone outside the summer until it reaches Glasgow (coming from someone who lived there for six years). Lancaster, Oxenholme, Penrith and Carlisle are all fairly small and insignificant.
On the ECML, Newcastle to Edinburgh is sparsely populated and virtually none of the other stops have big populations until you reach London. Outside of London traffic, it's mainly people changing on to the ECML due to a lack of direct trains 
Yes, I get that it links lots of different places, I just can't understand why people are so fascinated with linking these places with a direct train (rather than hundreds of other combinations of routes).
As pointed out, they are the biggest two that aren't linked and serve other decent populations on route- Sheffield, Leeds, Bradford catchment area and some of the smaller places draw in a lot of tourists.
I could draw a line on a map that serves much bigger places than the ones that you want to serve (Nottingham to Glasgow wouldn't serve anywhere of significant size between Leeds and Motherwell) .
That's no different to the WCML between Motherwell and Preston, or the ECML along a lot of its route.
You've said yourself that a "mainstream" of enthusiasts want this service, but where's the beef? What's the main selling point (other than it being relatively scenic)?
Wait... we need a direct Nottingham to Glasgow train because people prefer direct services, but people won't use the existing service from the East Midlands to Glasgow because it's not fast enough?
It's direct from the East Midlands; if you live in Derby or Chesterfield. It's not direct if you live in Nottingham, is it? And it's still an appalling service that is crowded, slow, circuitous and expensive.
Which way do you want it?
Well, there's a direct train every two hours from Glasgow to Leeds, which is the best service that it's had in as long as I can tell - possibly ever?
How much more frequent does it need to be?
It's awful, infrequent (2 hourly is not frequent), overcrowded, expensive and circuitous.
I remember doing Coventry - Nottingham in Central Trains days vie Nuneaton and Leicester.
Coventry is a reasonably sized city, about an hour's drive away from Nottingham (fifty miles?), but I don't remember seeing demands for a direct service (yet Glasgow, almost three hundred miles away...)
Ah, so Nottingham to Glasgow should exist because people prefer direct trains, but Nottingham doesn't need direct services to other places as long as there's one change?
It's long distance passengers who are less inclined to change, due to luggage etc.
You can do Nottingham to Glasgow by changing at Derby, Chesterfield, Sheffield, Manchester or Leeds - which surely gives a number of options - so why does Nottingham to Glasgow require a direct service (because a same-platform-interchange at Manchester Piccadilly isn't good enough?) whilst Nottingham apparently doesn't need direct services to other places in the Midlands?
Well, I see comments that a Nottingham service would need to go to Glasgow up the S&C because the WCML is too full, yet apparently the northern end of the WCML isn't very full - yet 99% of the passenger services from Preston/ Lancaster/ Penrith to Carlisle continue up the WCML to Lockerbie/ Carstairs.
Freight will be dwindling away on the WCML over the next decade too.
When Brighton is mentioned, the northern destination never seems to be settled upon - it's more of a "why can't we have some different services and a couple of randomly timed trains a day to Kensington Olympia like BR did" rather than agreement on whether a Brighton service would go to Manchester or Leeds or anywhere else.
...but if the WCML is so busy then how are you going to run from Carlisle to Glasgow? Up the GSW?
Much of the freight on the WCML is going to be questionable over the next ten years, as "traditional"/ heavy industries decline.
It's certainly no crime - I just can't understand the continuing fascination for linking these two places.
Personally, I just think you're being facetious and jumping on this for attention/the reaction. Being from Sheffield, are you happy with your links to Glasgow, Carlisle, Leeds, Bradford, or Nottingham?
Sadly the replies I've had so far aren't shedding any light on why these two cities deserve a direct link (whilst lots of potentially bigger markets from each city have no such link).
I'm beginning to think that so many people suggest it because so many people suggest it (rather than any real need).