It would need more units, some timetable tweaks, and a new platform at Leicester but an improved service could be:
Corby switches to a 2tph St Pancras to Leicester - Luton Airport, Bedford, Wellingborough, Kettering, Market Harborough, Leicester.
- Keeps the Luton Airport Express service and means from North of Leicester it's a one-stop connection (Leicester) to get to Luton Aiport, Bedford or Wellingborough (currently two changes)
Nottingham services become first stop Leicester then
- 1tph Loughborough, East Midlands Parkway, Nottingham
- 1tph Alfreton, Beeston, Nottingham
Sheffield services remain first stop Leicester then
- 1tph Long Eaton, Derby, Chesterfield, Sheffield
- 1tph Loughborough, East Midlands Parkway, Derby, Chesterfield, Sheffield
Fastest Brighton-Bedford service (2tph) extended North to serve Wellingborough, Kettering and Corby, possibly with a new Irchester and Rushden Parkway station on the Slow Lines on the old Irchester station site.
I know extremely unlikely to happen and the biggest "loser" would be Market Harborough switching to connect. But if you had 12-car EMUs, 2+2 seating and First Class is it any worse off really?