Ballarat, bendigo, seymour, warrnambool and swan hill will all go SG at some time. SG vlocities are just as likely to make it happen much quicker then you expect. At the very least there will be an SG line that will parallel the BG in those areas in time purely for making the freight connections.
Don’t delude yourself. The chance of an SG line being built parallel to any of the lines you mention is absolutely tiny, if not zero.
Ballarat: never, cf. interface with suburban, Geelong, Airport services etc.
Bendigo: not in the next 20 years. Despite what some people on here and John Hearsch wish, the Clarkefield diversion is dead, and Bendigo will be sharing tracks with other BG services for a good while.
Seymour: possible, but there will have to be a solution to the lack of SG capacity into Melbourne.
Warrnambool: never. Huge expense for absolutely zero gain.
Swan Hill: see Bendigo. More likely scenario is cessation of freight altogether. Only way you’re getting SG is with some sort of funky link via Inglewood, I.e. snowflake’s chance in hell.
The essential problem now with gauge in Victoria is that the low-hanging fruit - isolated, lightly trafficked lines without passenger service - is gone. Anything else from now is going to have a serious impact on commuter-belt passenger services, unless you want gauge islands, and many of the available options are hopelessly entangled with other lines.
The other question is of what benefits, if any, exist to further standardisation. Apart from everything west of Ballarat - inevitable in the medium term really but relatively small in the grand scheme of things - standardising any combination or part of Warrnambool, Swan Hill, Echuca (either way), or Bairnsdale is going to seriously jeopardise the viability of the remaining BG freight. No government will want to be opening a shiny “upgraded” line to, say, Deniliquin via Seymour while the papers yell about thousands of extra truck journeys being created by the cessation of all BG freight.
Add the fact that the last two standardisation projects have been a PR nightmare and I don’t think there is any political will to make the 165mm move any time soon in Victoria.