HSTs aren't permitted south of Reading unless the bogies are of the Short Swing Link variety.
If the HSTs will finish their days on Cross Country it will be between Plymouth and Edinburgh via the East Coast. The Voyagers will be needed to strengthen the remaining services and I would suggest to provide a third hourly path between Birmingham and York (via Leeds).
Electrification in thwe Northwest will see the end of the Pacers not IEP.
I would like the above to happen, once East Coast cascade their HSTs as FGW HSTs are spoken for
Penzance/Plymouth - Edinburgh/Glasgow/Aberdeen (HSTs with a few doubled up Voyagers) 1tph
Bournemouth - Manchester (Doubled up Voyagers) 1tph
Southampton/Reading - Newcastle via Doncaster (single 220s) 1tph
South West/Bristol - Manchester (single 221s) 1tph
NEW Cardiff - York via Leeds (220s) 1tph
AMMENDED New Street - Derby 2tph (170s)
New Street - Leicester (170s) Halts service 1tph
New Street - Stanstead Airport (170s) 1tph
There will not be enough HSTs cascaded to run every NE - SW service as HST, but will enable many services to be doubled up, as detailed above.
Not perfect, but working with what we have got