I don't know enough about the underground geometry of this, but would it be possible to get into Snow Hill? Perhaps this might finally provide the excuse to rebuild the line to Wolves Low Level and provide an extra link running right through the West Midlands, also conveniently giving an alternative route between London and Aberystwyth.
Interesting to see my old thread revived.
The approach to New Street goes directly underneath Moor Street station... so before you know where you are you're stuck with the 2 track Snow Hill tunnel - which of course has platforms at the Moor Street end.
Of course theres options to reinstate whats now platform 5 at Moor Street, and I think there is just about room for a platform 6. If the through platforms were not used at Peak times it might be possible to create more paths, and then reinstating *a* Snow Hill-Wolves link could allow for Marylebone to Liverpool, Wrexham, Chester (etc) services, freeing paths on the WCML and thus through New Street.
Of course an easier way of freeing paths in New Street could be via the Camp Hill chords, such as the Herefords (if my geography is right) or if some splitting of currently through XC Turbostar services were considered.