TL;DR - It won't matter what you kill on the Island anymore, only what mobs were on the Island, difficulty level (Normal, Heroic, Mythic, PVP), and how quickly you've completed it (?? unsure of this very last part).
In Tides of Vengeance, all players on an Expedition will have a chance to receive cosmetic rewards based on the island’s inhabitants, rather than which enemies were specifically defeated. That chance will again be higher on higher difficulties, but there will be no need to strategize around invaders. The most efficient way to receive cosmetic rewards will be whatever tactics cause you to complete the Island quickly, on the highest difficulty that you’re comfortable.