Some musings on your examples:
ECML
Newark to Lincoln - doesn't make sense for the one train a day that would take advantage of it, you'd have to electrify the line to Nottingham.
Doncaster to Hull - is (or at least was) getting wires - it's currently getting bi-modes instead.
Newcastle to Sunderland - could get interesting (in the sparky kind of way!)
Edinburgh Aberdeen/Inverness - are getting wired in the (very) long term.
WCML
Crewe to Chester - should be to Holyhead, no?
GWML
Cardiff to Fishguard - again, difficult to justify given the frequency of service.
Welsh Mainline up to Fishguard - As above
GEML
Norwich to Cromer - there's a strong case for this one, though somewhat diluted by GA getting bi-modes.