I don't think that another Manchester / London service is really needed. However, if a third "fast" path on the Hope Valley becomes available, I would suggest that a Manchester - Sheffield - Derby or Leicester service might be useful; south of Sheffield this could call at Duffield, Chesterfield, Belper, Derby, Long Eaton & Loughborough.
Alternatively, extend the current Northern Manchester / Sheffield stopper to Derby or Leicester. Would not exactly be fast, but would offer new through journey options instead of having to change at Sheffield.