I am very late to this party but my tuppence on the matter is that the main flaw with CrossCountry's current setup is all the straggly little journeys that occur all over its route. A train can be dead from (originating at) Plymouth to Taunton, then wedged out to Temple Meads, at a comfortable level of capacity from there to New St, busy to Derby, empty to Sheffield, absolutely full and standing to York, and then hardly a soul on it up towards Newcastle and Scotland. Some Manchester to Bournemouths are more than easily covered by 4 cars until Stoke, when it resembles the stereotypical Indian railway as far as Amazingstoke. Half of the Reading-Newcastles are dead until they hit Birmingham. It is exceptionally difficult to win when the cards are dealt like that.
A brand new fleet of 7/8/9-car IEPs is the dream were money no object, but the fact of the matter is that aside from a core around Birmingham (say the Manchester / Sheffield-Leeds / Bristol / Reading polygon) and maybe 20-30 minute legs here and there nationwide, they would be deserted; hemorrhaging money and resource.
The solution isn't adding 'fast' Birmingham-Leeds or Birmingham-Reading, serving only Derby and Sheffield, or just Oxford: the existing infrastructure struggles to cope as it is without wedging in more half-empty 8-car IEPs from Birmingham to [station] onto the graph because someone has to stand from Leamington to Oxford: Bristol to Birmingham averages 60mph and the lines to Derby, Manchester and towards leamington deal with a myriad of local services, freights, depots and flat junctions, there simply isn't the resource to start throwing even more
trains around.
I would err down the side of getting XC down to a core network of Birmingham-Edinburgh (maybe even Newcastle once TPE et al turn up) / Manchester / Bournemouth (Southampton?) / Plymouth, and whether by ticket restrictions or other resource (hell of a lot easier for a 11-car 390 to suck up Coventry commuters for 20 minutes than a 4-car Voyager, or cascade DMUs to the West to aid additional Taunton-Bristol provision etc.) try and keep local hop journeys away and leave the majority of the provision of CrossCountry to those trying to use it to get cross-country.