Imperial Japan conquers the Pacific without pissing off America (somehow) and the Allies resort to setting up guerilla groups to fight the Japs, like a kind of Lawrence of Vanuatu affair. The Pacific islands are divided by Allied politicians into Melanesia, Micronesia and Polynesia. Not very plausible, really.
The thing is, Melanesia is really more of a Western geographical construct than a cultural thing. If you ask a Fijian what he's got in common with a Papuan and he'll look at you as if you're crazy. That's the problem with far-flung island groups - I've no idea how Kiribati and the FSM manage, and they're pretty much arbitrary aggregates of individual atolls and island groups drawn up by people in ties and offices in Europe. And as I understand it, there's no love lost between the Solomon Islanders and the Ni-Vanuatu. I mean, yes, there's the Melanesian racial classification, but: A) that doesn't include Fiji; B) almost all of them (as in, 11 million out of 12 million total) are citizens of Indonesia, in the Moluccas and western New Guinea, so you have to deal with the Dutch or the Indos before you can detach them; and C) it only really matters to Europeans - and mostly to 19th century Europeans at that.
So yeah, something with Japan wiping the colonial slate clean (including Indonesia) is probably a good bet.
FYI, bumping your threads after half an hour isn't a good idea.