Clearly what's needed is to bring back the old Wolvercote/Yarnton to (approx) Oxford Parkway avoider - which crossed the (Birmingham) mainline at height -

. Wouldn't the folks of Charlbury love a competing line into Marylebone. It was a war-time addition (WW2) for freight to connect the Cotswolds line to the Bletchley line. It's clear enough on Google Maps - in parts - but no sign of it where it crosses the mainline. The viaduct over the canal is still in place, although a roundabout and the A4260 have some of its alignment, and the A34 crosses it. Alas I don't see Chiltern picking this up so late in the privatized railways game!