I would expect it's more likely to keep the Kina, if they retain good terms with PNG - which so far as I know, they seem to have. I could see AUD or NZD being a possibility, being relatively close geographically as well as having Commonwealth status.
Well, I try not to get political on here. But sometimes it's unavoidable so it's trying to appear neutral-ish. But the US does seem to be a bit more …unpredictable… compared to usual so I can see why people may be less willing to adopt it.
I know what it's like living through chaos first-hand though. I'm British & I raise you Brexit. 😛
I don't think USD is perhaps as widespread as you'd expect within the public domain. It's certainly the largest reserve currency by far, but I think only 4 other nations openly use USD (Marshall Islands, Micronesia, Palau, & El Salvador) as the official currency. Then about 5 more use it as a official secondary currency alongside their own issue, and maybe 5 more unofficially. Something like that.
But given that three of the four official users are (relatively) local neighbours, I do think USD has a chance.
Though I do hope they make their own designs. Always exciting to see new things out there (besides modern USD issues are just so… bland).