best options I think at the minute are probably
Leeds-Glasgow via S&C. Have the engines to do the climbing & the top speed to do it quickly if linespeed could be improved (it's only 110 miles Leeds-Carlisle, if you could average 60 you could raise Glasgow in under 3.5)
Capacity increase on STPE, either as 6-coaching the Cleethorpes trains, or potentially adding a second South TPE service through to Lincoln via Worksop, Retford & Gainsborough. Half-hourly Sheffield-Manchester would certainly be a bonus.
Nottingham-Liverpool, replacing the 158s there, allowing the 158s to be used in East Anglia. (I'm against single 158s Nottingham-Norwich for the simple reason that they can get extremely busy east of Peterborough. Personally I think a minimum half-hourly service is required Peterborough-Norwich, Peterborough-Cambridge and Norwich-Cambridge.