I've gone on record a few times saying that I would run somewhat closer to the original GCR route, bypassing Birmingham for now, concentrating instead on relieving the East Midlands, Sheffield, Leeds and Manchester with a 'Y' route splitting at Sheffield and the two arms reaching to Manchester and Leeds, extending along the 'classic' lines to Edinburgh and Glasgow. This would be a 6tph, 200mph service (somewhat more reasonable, I don't think that anything beyond that is even possible); backed up by an additional 6tph, 140mph service that would use parts of the current Chiltern route to Aylesbury, a reopened Brackley Central (to placate Chiltern opposition and provide a big commuter area with a link to London sixty years after it lost one) and then fanning out to Oxford, Stratford and Birmingham Moor Street. New Street would get some extra services, diverted from Manchester, while Hull would get an hourly service, diverted from Leeds. North of Brackley, XC would be able to use the route as a short-cut from Banbury to Sheffield, potentially saving an hour. There could also be links right through London to Kent. Finally, Chiltern could extend up the slow lines (i.e. the GCR trackbed) as far as Rugby. The idea is to concentrate on sorting out the MML corridor, which is still dependent on HSTs at 110.
Heathrow would be the next priority, although the line would be intended to go right through to Reading, Bath, Bristol, Cardiff and Swansea, possibly with a southern arm to Exeter and Plymouth, including wires to Penzance. This would be plugged into the GW lines at Old Oak, Reading, Didcot, Swindon, Wooton Bassatt, Bristol Parkway, Newport, Exeter, Cardiff, Plymouth and Swansea so effectively that it would have the same effect as quadrupling/sextupling the old line, thus spreading the benefits to Worcester, Hereford, Cheltenham, Newbury, Fishguard and all sorts of other off-route places.
Eventually, all the major routes out of London would receive the same treatment. This allows trains to use the old stations and spreads the benefit much more effectively, with the added benefit that infrastructure trains can run via the old lines worksites along the route. However, by running alongside rather than on top of the old line, there is no need to shut down for upgrading.