There is not just bigger model planes to consider, but look at Qantas current 737 fleet then look at whats on the market now with 737-8 and 9 series. Without checking actual numbers, probably 50+ more bodies per plane. Likewise the older 767's.With regard to the narrow-body fleet, the problem is that Qantas has standardised on the B738 as its domestic mainline workhorse. While it could certainly get good use out of B739ERs, the fact that it hasn't I think is quite telling. I personally think that the Australian domestic market could probably use the A321 well, but I also think that the nature of the domestic market is that we need a mixture of a workhorse that we can do most of the jobs (additional capacity made up through frequency), and a big gun. Traditionally with Qantas, this has been the B737 and B767.
I'm inclined to think that while the A321 and B739ER could probably be used well in Australia, the economics of standardising on the B738 and A320 seem to have prevailed - nothing wrong with that, of course.
With the wide-bodies, however, we open a whole new can of worms...
The main issue for the bigger planes is lack of dual loading and unloading air bridges, but resolve this which I think we will see and with increasing demand elsewhere in Australia, we will see more larger planes used by Qantas and Virgin in the 2020's for domestic usage, other countries can use 747's for 1hr flights due to demand. The A380 on medium haul routes struggles on economics due to bad turn-arounds unless they can load in similar time to the competition planes. Hence why Emirates built a purpose built terminal for A380's at Dubai as Europe-UAE is a popular medium haul route that has tripple airbridge loading capacity. Basically they can load an A380 is same time as a B737.It's not just aerobridges, though that has its role. Cargo loading is also problematic, and these two factors slaughtered the A332 on domestic runs the last time they were put on Cityflyer (they took 80 minutes to be turned around, whereas a B763 could be turned around in 45), before being swapped out to transcon and replaced on Cityflyer with B763s.