Here’s my prediction for the order of the islands and trial captains.
We know from the charm that the player will start on the yellow island, then go to the red, the pink, and finish at the purple.
Using this information we can predict the island captains.
For melemele it has to be Lana, as she doesn’t seem to fit with any other island.
Sophocles has to be next because the location he is seen in in the trailer appears to be the observatory from the big Island, which is the second in the sequence.
Mallow is next because it is confirmed that she is on Akala island, the pink island, real life Maui (to the right of melemele), which is third in the series.
And that leaves Kiawe on the last island, poni, the purple island (the left most island,based on kauai). This also fits because Kiawe is interested in dances and this is the most secluded island, most in touch with the tribal peoples who first inhabited the islands.