Rolling stock wise, assuming GBR consolidates the operations of various TOCs, mainly operations staff with overlapping route knowledge, it would be most advantageous to have an 80x derivative given that LNER, TPE, EMR, Avanti and GWR staff already have the knowledge of these units.
Given what has been said about the economies of XC the only way they can increase capacity and decarbonise would be to order rolling stock to cascade enough of GWR and LNER’s Class 800s to XC. Finish GWEP and order EMUs for the London to Oxford, Cardiff, Swansea and Bristol services and more 897s for LNER.
That would mean three existing TOCs would gain new (to them in the case of XC) trains and more capacity, two of which would serve London, making the economic case much stronger.
Whether this would be possible within the confines of the IEP contract is something that I’m not too familiar with. With some more comfortable seats, softer lighting and more reliable fuel pumps it could work quite well.