GBR is planned to be regionalised, mapped to the NR Regions (which might change, of course).
But you could imagine a situation where London-Birmingham was in a single Region (ie containing what are currently Avanti, LNW/WMR and Chiltern services).
It will be up to a GBR pricing manager to devise a system which works on all 3 groups of services and maximises total revenue.
There will still be a need for a premium on fast "Avanti" services and incentives to use spare capacity on "LNW/WMR" and "Chiltern".
The result might not be that different from now, but it might be marketed differently.
Would we see a coordinated definition of "peak time"? Probably not, the differences between VT/LM/CH are deeply ingrained.
Commuting revenue on the slower services, and long-distance revenue on Avanti, will still need to be protected.
Meanwhile, next door, the Western Region will be doing the same job from Paddington, with possibly different results, and the routes overlap in the south Midlands.
The successor to XC will be in there as well, complicating the scene (and not Regionalised?).
It's not going to be an easy job.