Rowville line, Melton electrification & airport line won't be done until the Melbourne Metro rail tunnel is completed. All 3 are planned to run through the MMRT along with the Pak/Cran and Sunbury lines.
Likewise, a Doncaster line won't now even be considered until Metro tunnel 2 (Clifton Hill to Fisherman's Bend to Newport) is built (which is sadly a long way off). This would allow the South Morang/Mernda line to run through Metro 2 while Doncaster & Hurstbridge lines run via the City Loop.
This is the first I heard of tunnel 2; and I went looking out of curiosuty. There seems precious info on it, but I've long thought that a tunnel under the water to the west via fisherman's bend would be a great addition to the system.