777X to replace the 744s from about 2022/23 I reckon. The 787 is a great machine but it's not a 744 replacement. I said that before QF announced them and the current fleet announcement has borne that out.
A320s for mainline gained serious consideration when JQ HKG was finished off. I reckon 75% chance the NEOs will appear in mainline colours from late 2018 onwards.
QF announced the 787s two years in advance. If they want any beyond the first 8 they'll need to firm them up around the end of this year. Expect them to be announced probably at the Full year results or AGM*.
* Presumes no WWIII or significant financial crisis.