Guess's on How Sasappis died.
Sasappis is the last ghost for us to learn his death. Sass is very private (undead) person, and we have been given no real/clear hints to his death. His ghost power (To enter into the dreams of the livings) could easily correlate to him being a story teller, and not necisarly "He died in his sleep, after catching a sever fever, that also caused him to have very strange dreams. Note I do not believe that Sass died of a fever. They would have incorporated some sign of it into his character. He would be more tired, feel more cold, possibly have a "unexplainable sweating problem". He also does not have any vibsiable gaping wounds, like Pete, Flower, or Stephanie, so that rules out death by attacker. So the question is how did Sass die
Now he could have died of some kind of sickness, but I do not think it is very likely. For one thing he is implied to have had a sudden death, just days before an important tribal ceremony he was planning/supposed to take part in. Correct me if I am wrong, but wouldn't most sickness take a period of weeks, months, years to do him in. A second thing again he isn't really sickly in the the show. All the cholera ghosts are flushed and still hacking. Issac will sneeze and mention stomach pains. But Sass seems fine, so if he has any kind of sickness then he has been masking it for 500 years. That is really sad.
He also could have could died if he went walk in the woods one night and fell from a cliff or out of a tree... but if that happened well that is kinda anti-climactic for how long we had to wait. I would rate that on the same scale as Thorfinn dying because he went walking with his helmet, through a storm and was struck by lightening. It is just a case of him not being as careful as he should have been. Not nearly an impressive follow up to Alberta or Hetty's more recently revealed deaths.... But you could make it more sad. Sass and his crush Shiki are implied to have died in the same year. Sass says that when they were alive, he was ghosted by Shiki after gifting a freshly killed deer to her family, as a jester of love. What if that was because Shiki actually went missing (i.e. died) shortly after, and Sass went out to search for her?
Another death I have considered was a food related one. A detail of Sass's character is is the most in love with food smells (the closest they can come to eating) of all the ghosts. He hangs out in the kitchen whenever Jay is cooking, consistently begs Sam and Jay to cook more of his favorite smelling food: pepperoni pizza, and I am honestly half expecting him to move into the barn once Jay's restaurant opens... What if Sass's love of food is actually a hint at his death? Maybe he had an allergy and accidentally ate something he was not supposed to? Or, maybe he had diabetes or some other dietary need, a crop or the meat from a particular animal, and Sass died when it was wiped out or disappeared. With so many people in the U.S. having dietary issues, and the countries known love of food, I could see the show choosing to write an episode on dietary importance, and the need for people from all walks of life to have access to healthy food.
My finale theory is one I have actually gotten off Tumblr, from the people who are fans of both the CBS and BBC series. Some people are suggesting Sass got bit by a poison snake or something and died from the poison venom. The puncture marks are small enough for Sass to be able to conceal with his clothing. Apparently something of the similar nature happened to BBC's Kitty? And I actually know a really good way to blend that into Sass's personality and a possible reason for why he became a ghost. Sass as mentioned loves stories and is a gifted story teller. But also as mentioned he is very private person who gets nervous about speaking in front of crowds. A week before the tribes festival Sass's father gave him an eagle's feather, which was believed to help bring confidence and courage. But what if Sass was still nervous/afraid afterwards. What as the days got closer got more more nervous/afraid, until he decided to do something reckless and stupid, to prove to himself that he could be brave. Something like walking through a more dangerous (poison animal infested) part of valley, telling himself "If I can brave this I will have no reason to be afraid of anything anymore". While that would not be the most exciting death, it would be very tragic, and it could set up "Overcoming the fears he had as a living", all of them, as Sass's unfinished business. Essentially, he would need to become a more rounded and confident person. Note that seems to also be Pete's unfinished business. Sass and Pete are roommates, and we have had at least a few really nice episodes exploring their relationship. It would be really cool if that continued to him and Pete having to teach themselves the same thing, but in different ways.