I have been contemplating a similar thought, more 80s/90s onwards along the London Extension
NSE days, I imaged the 117s being replaced by the 165s/166s to Aylesbury or even to Verney Junction.
Diesel Hauled to Rugby and beyond I assumed would be a mix of the 31s, 47s, or 50s, depending on where termination was Manchester, Sheffield
A more Modern Approach would be the Chiltern 165s and 67s & 68s, based on a what if they reopened the line as they intended to many years ago