It seems to be generally accepted that the Midlands Main Line is ‘full’ (at least Monday-Saturday) south of Bedford with 6 EMR services plus Thameslink partially on the Fast Lines including crossing moves.
Agreed. That's why I don't think adding loads of extra services is realistic. However, it very much is possible to run the existing pattern (6 tph EMR services) at full length using the existing 4 platforms. The entire connect service runs from platform 1 in the current timetable, so leaving that as-is, that gives 3 platforms for 4 tph.
IC services arrive at (current timetable)
.08 Sheffield
.27 Nottingham
.38 Sheffield
.57 Nottingham
IC services depart at (current timetable)
.03 Sheffield
.06 Nottingham
.32 Sheffield
.35 Nottingham
My proposal:
The .08 arrival forms the .35 departure
The .38 arrival forms the .06 departure
The .27 arrival forms the .03 departure
The .57 arrival forms the .32 departure
The .08 arrives in the platform the .03 departed from
The .38 arrives in the platform the .32 departed from
The .27 arrives in the platform the .06 departed from
The .57 arrives in the platform the .35 departed from
In this way, you get 4 tph cycling through three platforms.
Note how no service has to share platforms, all have sufficient turn-around time (25+ minutes), and this way every service can be full-length, without changing the timetable.