Okay, so how about getting a couple of Meridians and running them in
VXC's slot between Birmingham and Coventry (divert VXC via Solihull,
swap the 323s on the Coventry line for the 150s on the Walsall Line
and extend them to Leamington), then either direct or via Northampton
to Milton Keynes, where the train can join the Slow Lines and either
run via Bicester Town to Oxford or via Aylesbury to Marylebone?

Via Aylesbury & High Wycombe please! Given the nature of the north-south
road network in Buckinghamshire, a service between High Wycombe,
Aylesbury & Milton Keynes could (if it were quick enough) attract some
car users and would generate its own traffic.

More pie-in-the-sky, but if such a service were run from Wycombe to
Maidenhead instead of London (relaying the Wycombe - Bourne End line),
then there is the possibility of running to Heathrow (using electric
traction from Maidenhead via Crossrail) and thus providing a direct link
from the WCML to Heathrow (not to mention easier access between Thames
Valley employment and Buckinghamshire).

Heathrow's probably out, but a Birmingham - Maidenhead service would
sure be useful. Perhaps combined with a second runway at BHX it could
attract people away from Heathrow. I've never managed to spot the
point where the Bourne End line branches off SE of High Wycombe
station. It hasn't been built over has it?