Yeah, the London to Liverpool locals were born out of NSE's outer suburban services, combining them with Central Trains services, and they form a genuine budget alternative to Avanti. Elsewhere, though, the opposite has happened - former Thames Trains services have sort of disappeared, because GWR now uses IETs on them, and they're hard to tell apart from former IC services. Separating them out again would be difficult, and I'm not convinced it would be worth the effort.
GWR is one of the thornier ones for sure, though it could be done (although to what benefit?).
Chiltern I don’t think is actually an issue, as HS2 will probably scupper their Birmingham visions, and one could certainly argue that the “London commuter” operation now goes out beyond Banbury.
LNwR again would be a case of splitting at Birmingham, and might actually give a bit more focus to Northampton which seems to be forgotten whilst the TOC looks at Birmingham revenue.
C2C, GTR and the southern region TOCs are no problem, and you could easily push NSE out to Norwich nowadays with a modern version of the “Network Express” brand.
So it’s only really GWR that is a problem. However electrify to Oxford and Berwyn, and you could probably siphon off those routes easily if a paperwork boundary was needed. NSE could quite easily reach Hereford, and perhaps Swindon/Cheltenham.