There are currently no concrete plans for electrification to Melton, let alone Bacchus Marsh. The works that are currently under way, are to provide duplicated track as far as Melton in preperation for more services on this line. To begin with, these will be diesel services.
Essentially, after the line is duplicated to Melton, it will then be easier for it to be electrified.
To electrify the extra 13km from Melton to Bacchus Marsh would push the cost up significantly. The line from Caroline Springs to Melton is mostly flat, whereas from Melton to Bacchus Marsh it is less so. Actually, Melton to Parwan wouldn't be too bad, but the section from Parwan Loop to Bacchus Marsh would need significant work to add a second track. This would be paramount before the line could be electrified.
The real question is whether their is actually any need for it. Currently most of the short workings on this line run to Bacchus Marsh, purely because that is where the stabling sidings are. There are in fact some services that run only to Melton, then either return to Melbourne or run empty cars to Bacchus Marsh and directly into the yard. Melton has never really had sidings to stable passenger trains, there were goods roads back in the day, but that isn't the same thing.
The reality is though, when short working PM peak services leave Melton, their aren't that many passengers left, The bulk of the patronage on the Bacchus Marsh/ Melton short workings come from Deer Park, Caroline Springs and of course Melton.
When full scale duplication and electrification occurs as far as Melton, there is no reason the Bacchus Marsh passengers couldn't use a bumped up Ballarat service.
Provided good timetabling, Ballarat trains will then not be required to make stops at Rockbank, Caroline Springs, Deer Park or Ardeer, so, all trains stopping at Bacchus Marsh shouldn't be too much of an issue.