There is very little space around Sunshine station itself for anymore platforms, but as others have said, this is fine for now*, as the Airport line can exist on the current suburban line between Sunbury line services.
Down from Sunshine things improve and there is plenty of space West of the current lines from past the new H.V. McKay Bridge.
I suspect the following is what will happen:
1. The Bendigo RRL line will need to be moved, where it currently diverges to the Sunbury line, it will instead slew further to the South West, and run clear past Albion and as far as to where the Sunbury line passes under the M80 Ring Rd, and this is where the Bendigo line should diverge back into the Sunbury line.
No residential property acquisitions would be required, although the old electrical substation at Albion may not be longed for this world.
There is enough space under the Ballarat Rd overpass to fit another track pair, Albion station will lose a fair chunk of its car park though.
2. In the now vacant location that used to be where the Bendigo RRL diverged with the Sunbury line, there will be a new diverge for the new Airport Line. This line will then run parallel and between the Bendigo and Sunbury lines, rising fast to climb over both the Ballarat Rd overpass and the Sunbury line and then it will descend to eventually run at ground level alongside the Albion - Jacana Goods line.
Two new track pairs will need to cross Anderson Rd, so this will require significant bridge works, the Southern most of which will need to be elevated to some degree to provide enough road clearance underneath. This will result in 7 tracks crossing Anderson Rd.
* While Airport and Sunbury line services can fit on this single pair of suburban tracks without too much of a problem, it remains to be seen what happens when Melton and possibly also Wyndham Vale sparks have to squeeze through here also. It would seem, that is being left as a problem for another day.