Then that really is difficult. If the WCML Upgrade hadn't been de-specced, then there would have been the underpass route at Nuneaton which would have allowed through running from Leicester to Coventry, and you could have got out of the south end of Coventry to Leamington, whence Oxford, Didcot West Curve, and Swindon to the West. But that would have needed some significant work at Coventry, almost certainly.
The alternative is one of those nice might-have-beens. If the Midland line from Leicester to Rugby hadn't been closed, then there would have been a very fast route south to Rugby (where additional work would have been needed during the re-building of the station). The real might-have-been bit is south of Rugby. If the railway north from Banbury had been built as planned to Rugby rather than taking a left turn and heading to Leamington ...
Oh, and one other. Build the BR-proposed Whetstone Curve, restore the GC as far as Woodford and then restore the link to Banbury.
But given where we are, I think the only real possibility would be an East Midlands route between Doncaster and Birmingham alongside the present one, using the ECML, a new curve at Newark, and the route via Nuneaton, all upgraded where necessary, of course.