IMO the most feasible expansion of Sydney's electrification would be Kiama - Bomaderry. Plans are in to provide a 4km passing loop at Toolijooa and a second platform at Bomaderry (https://www.transport.nsw.gov.au/projects/current-projects/kiama-to-bomaderry-project
). The Govt claims this will double service capacity. Perhaps they use the momentum to keep going and upgrading the line further. After all, it's not a long branch.
I also know they're already planning duplication in the near future for parts of the SCO (between Unanderra and Dapto IIRC) and various alignment changes between Helensburgh and I think Thirroul over the coming decades to speed services up. I can't remember where to find it, but it was in a government report.
Extending the OH to Bomaderry (and some height clearance upgrades) would allow the D Sets to operate the full length of the line, making services more efficient and reducing the need for empty running when the singular DMU at Kiama needs to return to Sydney for maintenance. I'd personally like to see the line extended to Nowra proper but I doubt it'd happen because it would be prohibitively expensive.
I can't see Hunter happening soon due to low patronage and the need to mix rolling stock with J and CAF sets. Similar situation applies to the SHL, the patronage just isn't high enough to warrant the cost. Moss Vale Electrification may come at some point but I don't see it happening any time soon. If the Hunter gets electrified eventually, it'd be to Telarah, not Maitland.
As for Canberra, priority one should be moving the station somewhere useful, adding an additional platform, duplication, more services, realignment to allow faster running. Electrification can come way later.