ATA also flew 738's from OAK-HNL and AQ flew 73G's on West Coast-HNL routes. WN would need ETOPS certification to run these routes. In addition, WN would have to operate red-eye departures, which is against their current philosophy. I'm sure that WN is crunching the numbers to see what it would take to make Hawaii work.
A perfect route for WN would be OAK-HNL. If WN were to start service, I would expect 3 or 4 late afternoon/early evening departures (e.g 4pm, 5:30pm, and 7pm) from OAK-HNL with red-eye returns (e.g. 9pm, 10:30pm, and 11:45PM). That would be the only way to optimize fleet usage and maximize connections.