Entirely, countries consist of small islands in the Pacific Ocean don't seem to be common for tourists' destinations.
There are tons of splendid beaches in the Pacific Ocean, and most of them are not bothered by overtourism. It means that there is no reason for them to dare to choose uncommon destinations. So, it would be difficult for least-visited-countries in the Pacific Ocean to get more tourists without doing anything. The only thing I come up with is to offer direct flight from mega cities with cheaper price.