Not sure what order they would die in but I most certainly would know how they would
Fluttershy-Gets mauled to death trying to befriend a bear
Pinkie Pie-Suicide, I have a headcanon that Pinkie has minor depression, but it would worsen over the years which results in her death
Applejack-Farming accident, quite a few people die from tractors
Twilight-Stress causing insanity.
Rainbow Dash and Rarity-?????