I suspect preliminary plans already exist for many of these suggestions, but adding them all together it might be easier to start a separate list of lines that aren't suitable for electrification! Looking at other countries that have electrified almost everything we aren't short of projects for 4 times as many electrification teams. Just short of the money trees to pay for it all.
I noted at the public inquiry for the Hope Valley Capacity Scheme that a new footbridge at Hathersage West (to replace a foot crossing used by perhaps at very most 10 people a day, with an alternative path not far away) is to cost £750,000 at 2016 prices and will be an eyesore because it has to comply with electrification specifications. Any further visual intrusion will be resisted in the Peak District! I understand a small team prepared a serious outline plan for that ultimate electrification - for it to be discounted due to the cost and massive blockades it would need to get it done, particularly through 3 long and wet tunnels.
I suspect a lot of the other suggestions in this thread may have been looked at already and will be on wish lists - and also on too hard lists like this one.