The WCML comes in sections in TS1 and would be much harder (or take longer) to make in TSW but I'm sure they will do some of it at some point
They've been talking to Virgin so I'd love to see this. If they could do the Euston to New Street route and double-license the Virgin Pendolino/Voyager and maybe a West Midlands Trains EMU to do the stopping services that would bring it bang up to date.
I wonder how far progressed any work with them may be. Might be worth them pushing out the route (if indeed, that's the one they're looking at) as a long route with Virgin trains, then adding new liveries/services as First take over. It's nice having options, like being able to choose between DB or EWS liveries on the GWR route as people will naturally have an affinity for one over the other (I always see EWS trains on the line near us).
Unlike with TS2019,and due to the distance (85 miles), I think Glasgow to Carlisle would need to be split into two sections: (1) Glasgow Central to Carstairs Junction, and (2) Carstairs Junction to Carlisle. Similarly, other parts of the WCML would need to be produced in shorter sections than those available for TS2019.
Not disagreeing with you as the limitation is dev time rather than route length. My question would be whether it's possible to extend routes in TSW rather than each route being individual as per TSx, ie could you have extension DLC to a route which are "more map tiles" rather then being a distinct route. This would allow "hub routes" to then be built upon rather than completely new routes in and of themselves. If this can be done then you could, over time, build up Glasgow to Carlisle or even further over time
I don't know if that will be possible, but I certainly like that idea. Let's hope something can be done along the lines of your suggestion.