I've thought similar for some time - or rather what I think would be better was if XC terminated at Edinburgh, with an enhanced "via Motherwell" service provide by ScotRail - at least hourly, and probably based on an streamlined version of the very poorly-timed existing ScotRail service via Wishaw. Its utterly bonkers that Motherwell has such a poor service to Edinburgh, that has to be left in the hands of XC to provide the peak-hour service, whilst Cumbernauld is now getting 2tph to Edinburgh. If anything, if it could be pathed, a 2tph Edinburgh - Carstairs - Wishaw - Motherwell - Glasgow C looks justifiable.
My vision would be a joint Scotrail/TPE service using a further build of these 397s. An hourly semifast from Glasgow to at least Newcastle, calling at Motherwell, possibly not Wishaw because I think the link is single tine and slow, Carluke rather than Carstairs (larger population), Kirknewton (for Livingston) and all the expected stations to Newcastle. Indeed if it leaves Edinburgh immediately after the .00 LNER departure there should be space for at least a Reston stop.
After a pause to let the .30 overtake, it could continue, calling at stations to York, using the slow lines in the Vale to call at Thirsk again allowing overtaking.
In a sane world, where the wires were completed to Leeds, it would continue to there, and indeed to Bradford.
This would release XC stock for its overcrowded core.