I agree with Darlo, Given that XC are woefully short of capacity and we shall probably not see wires between Reading or Bristol and Brum, or from Lichfield/Tamworth to Leeds for several decades I would say concentrate them there, even if it means timetabling that route slightly slower to allow for the stock on it. Make sure that any replacement fleet has at least as many seats (and is as comfortable) as the trains it replaces.
Nice idea but how do you plan to amend the XC route to cope with trains that can't accelerate as well as Voyagers?
For example, the Birmingham - Leeds - Edinburgh train (currently ex Reading/ Southampton due to the Derby blockade, but normally ex Plymouth) passes Aldwarke Junction north of Rotherham at xx:30, getting to Leeds half an hour later.
At xx:34, the Northern stopper gets out of the Rotherham loop and onto the mainline at Aldwarke, stopping at every station to Leeds, taking around fifty five minutes to reach Leeds.
Run the XC service with trains unable to accelerate from Sheffield on time and you'll find them stuck behind the Northern stopper (as can happen when XC have been delayed further south), suddenly your train is arriving in Leeds half an hour late. And good luck fighting for a path out of Leeds with your delayed train, over the busy section towards Neville Hill.
Ah, but why not delay the Northern stopper to accommodate the slower XC services? Well, these are carefully timed to cross at Rotherham with the Hull - Sheffield stoppers (due to the single track Holmes Chord). Run the stopper at other times and you'll bump into Kings Cross - Leeds services north of Moorthorpe (given that they are about ten minutes faster than the Northern service over this bit of route, you have to be careful about when you let the Northern service onto the main line).
And this is just one local example of one place where an XC service is going to get into trouble if you run it with "slightly slower stock". See also the route north of Bristol (where XC can get from Temple Meads to the junction outside Gloucester in about half an hour, compared to the GWR stopper that takes almost fifty minutes), so adding a minute here and a minute there to accommodate slowly accelerating stock is going to mean getting regularly stuck behind a Bristol - Gloucester stopper. Or maybe the frequent Cross City services in the West Midlands, or the Paignton stoppers from Newton Abbott to Exeter, or the ScotRail stoppers from Drem to Waverley or something as delicate as finding paths through New Street...
There's no simple way of tinkering with the XC timetable to accommodate long HSTs (and that's assuming that we can get sufficient Mk3s upgraded to be sufficiently accessible for service in 2020 - given the problems with upgrading old trains I'd not be hopeful...).
Nice idea, but I'm out.