You would have to cut approximately 30-40 minutes off the post HS2b timing to make that feasible.
There are several routse that could be considered, a new HSL North from the vicinity of Wigan would probably be the most heavily trafficed line, but would have a relatively small time saving.
Next up would be new track in scotland, but the slowest parts of the route are north of Carstairs, which means that each km of track built would only improve the journey times to one of the two Scottish cities, so economically it is not particularly attractiev because the total journey time reductions (in terms of passenger-minutes) per km built are lower.
There are also occasionally proposals for short sections of track in the vicinity of Morpeth and Durham on the ECML, at which point you could route via the Leeds branch and York.
Whichever is best I am not sure, but I think line north from Wigan is the most likely to be funded, followed by ECML improvements.