I think that potentially fatal diseases would be fun if they were relatively rare and not named after terrifying real-life diseases. Like maybe if the giggly disease goes untreated for too long, you die of hysteria. Stuff like that.
As someone that's struggled with anorexia, that's last thing I want to see in a game I play for fun and escapism. Simulated eating disorders are not my idea of a good time, lol.
And I don't know if cancer is the best idea either. I get that some people have had family members that drowned or died in a fire, but it's different from cancer. You can't give cancer a whimsical twist. Do you knew what I mean? It's just.. different. Like there's a reason why in "A Series of Unfortunate Events" the parents died in a fire, not from cancer. It's still devastating, but it's far enough from reality and fantastical enough for a children's book. I dunno, it's just an example. I hope that I don't sound too bias or like I'm being outright dismissive. Yeah. It's just that cancer is more common and affects so many people. There's nothing entertaining about it. Maybe in a game with a much, much darker tone. I'm not saying that everything needs to be light and happy, but certain things would just kill the fun, playful mood of The Sims.
I definitely agree with the idea of diseases that could kill our sims, though. I just think there should be a limit of how much of the dark parts of the real world are brought in.
"The Sims 4: Get Cancer!"
It just doesn't have a ring to it.
Edited to say that I'm not saying that dying in a fire is fantastical and unrealistic. It is a real part of life and an absolutely devastating part of it. There's just a certain tone that makes it different. I don't want to offend anyone or make light of anyone's experiences. :)