The Barrie line is also a relatively quick win where electrification would strengthen the case for some infill stations - particularly Caledonia, Highway 7, Mulock, and Innisfil/Alcona. It's also not like Metrolinx can't make headway on multiple corridors at once.
I cannot see where there would be any obvious choice besides LSE/LSW - ridership, potential to grow ridership with better/faster service (west to Hamilton and beyond?, east to Bowmanville and beyond?), population densities. After that, additional lines could be added to build on service and ridership improvements.
And if you were next extending electrification to Milton, Guelph, Kitchener and perhaps London, would you not look at that in conjunction with VIA HFR to points beyond? You could then look at the same model working eastwards , adding improved HFR towards Kingston and beyond.
I think HSR is a pipedream, one the politicos will love to talk about. But electrified regional GO, along with continuing incremental HFR build is a possibility, and one more palatable when balancing the $ needed to invest in other public transit (TTC), Pearson, Billy Bishop, and road networks.
CN/CP are working entities that are not handing over track for no reason at all, and if anything should be seeing and recruiting new and expanding services. However, they have corridors that are largely rail friendly and follow population densities. Those are the routes HFR needs to build on, and if they cannot be acquired, then GO/HFR should improve as either as joint/shared capacity or twinned capacity.
Unfortunately you have multiple levels of government and agencies involved (and even a new one apparently) and I am sure that will be a hurdle of Olympian proportions.
i first remember picking up my parents from the Clarkson Road GO stop when GO used ONR equipment. Obviously in terms of NAM rail we have come a long way, and we continue to seem to have a more pro transit, pro rail attitude towards steady improvements and capital investment then other jurisdictions. But drive the highways of the GTA any day of the week and you know we are only reaching a fraction of possible ridership (and I would include bus and all possibilities of higher average speed bus transit)