If we're doing crystal ball stuff to CP6, what might wwe expect the next expansion of wires on the Southern (South West division) to be? further west to Weymouth? Creeping north on an assumption that the 455s.
Also, Basingstoke won't be the only changeover point in this round. Eastleigh and St Denys will have to be as well, and Southern are going to have make sure they're using DV stock into Southampton
My Guess would be Woking to Basingstoke, as with only 25 non railway crossing and several of these being either natural high or have been re-built in recent years. Likewise the signalling was re-done recently and with the footbridge at Farnborough station replaced last year, one would hope they designed for wires when they did these works. Which would mean limited works required to enable overhead cables.
It also means that a longer section of track is overhead wired but without having to convert the suburban trains (which terminate at Woking). It would also mean that the branches to Guildford/Portsmouth and Alton could then be added to the list.
Reading to Gatwick would be nice, but probably is down the list a bit when the tracks that it links to are being done as the GW franchise would be keen not to have to stable diesel trains at Reading in the future just for this route.
Southampton to Weymouth is likely especially if 110mph running is be possible.
Also possibly Southampton to Salisbury and back to Basingstoke as a diversionary route for freight and likewise routes to Westbury.
Otherwise I would expect to see the outer edges and more rural parts of the network being converted first until it starts to have a bearing on new rolling-stock ordering when whole lengths being done quickly.