I'm not against some of the other options being proposed, but my fear is that Omar, Honolulu or Brisbane is unlikely to get the crowds or generate the hype that Auckland can for this fixture.
The large Tongan community in and around Auckland, plus the fact we would only need to sell 30,000 seats does make it the safer option, but in order to ensure that 2018 isn't the last time a Tier 1 nation plays Tonga until 2021, we need for the game to be a success.
Auckland also works relatively well for broadcast into the Pacific and back into Australia and arguably the NZ media is more likely to get excited by this match up than the Australia v NZ match (unless the NZ players in the NRL have a great season)