The B787-9 is significantly larger than the B767 (virtually identical measurements to the A330-300) and is optimised for LR/ULR routes. The engines are significantly heavier than B767 engines and probably wouldn't handle the large number of cycles incurred with short range domestic flying. I can see the B787 gradually replacing the A330 on QF routes to Asia (probably the B787-10 model) but it would not be a good choice for domestic ops.
That leaves the choice of A321 NEO or the Boeing NMA (available from 2025) as likely options for domestic flights that require a larger capacity than the B737-800.