The reality is that a town-to-town railway is only going to give rise to a BCR that's considered "good value" by today's standards if it enables connectivity between two poorly connected big attractors (i.e. Oxford to Cambridge), and/or if it serves the market also served by a road that's at least a dual carriageway A road, if not motorway, or else a highly congested road.
Northampton is a very large town that is currently disproportionately poorly served by the railway (thanks to the indigent landowners at the time of the WCML being built...). The roads have adjusted to the growth of the town but there is still only one station with slow (if cheap) north-south links. It would do well to have better regional connectivity.
As much as I have vested interests in it being built, you just have to look at the map to realise that a Northampton to Banbury line won't be built in a month of Sundays under this country's contemporaneous approach to rail investment.
Were it ever to happen, it would have to go via Towcester and Brackley to be even remotely viable and politically acceptable. That in turn would make it quite slow and uncompetitive for Northampton to Banbury journeys (especially seeing as it would scarcely justify more than an hour service). Even less so for journeys from further afield (Kettering/Peterborough etc.).
Even then, Towcester and Brackley would probably be better served with links to major high-end employment centres such as MK, Oxford and London. And so the circle will never be squared, because you can't serve both needs with one line, and two lines are even more unthinkable than building even just one.
In short, this is all just a load of pie in the sky thinking, much as I'd love for it to happen. We'd need to reduce costs by roughly an order of magnitude, and demand lower BCRs from infrastructure investment, before it could ever happen. Unless there's a crucial by-election that can be won by promising that they will start
to think about doing it