Once again, no justification.
There's no capacity to run more between MK and Northampton as has been explained by
@The Planner on several occasions, usually when extending EWR or the Southern MK - Croydon service comes up.
Add in even if you got to Market Harborough there's no capacity between there and Leicester - EMR in the May timetable will be running 4 tph up there add in some freight and the paths don't exist.
Lastly, most of the journeys you're suggesting can already be achieved with a single change (Birmingham, Tamworth or Nuneaton) - and nobody has yet proven there is a huge demand to travel from Nottingham to Oxford.
...
But you haven't demonstrated there is the demand to travel from MK or Northampton to Leicester or Nottingham - that's the key.
Just because the M1 takes that line, doesn't automatically mean there's a justifiable or viable railway route there. You could equally argue that the M1 links London and Leeds, but the main rail route between the two goes nowhere near the M1 and the line which actually shadows the M1 has always struggled to justify a London - Leeds service on its route.