@mizal This is my very best idea:
An epidemic has spread across the world. Nearly everyone is dead, and the remaining survivors don't know of each other's existence. They must survive their own insanity and PTSD while searching for any other survivors, whom they don't even know of. They must battle hallucinations and the crumbling environment to survive.
You play as several people throughout the story, who's minds have each been broken in a different way. For example, one character you play as in one part of the story may have developed depression, while another you play as at a different time may have developed paranoia. In the epilogue of humanity, they must try to recognize what is real, and what is their own imagination.
The characters may cross paths, only to find themselves separated by collapsing buildings. They could end up traveling through the same environment, though only the reader would have any idea. As the story would be told in a first person perspective, the reader would not be able to know if they can trust what the character they are playing as is describing.
The twist to the story is that you can't save everyone, because the actions taken to prevent one character's death will lead to another character's death, therefore the 'best' ending doesn't exist, or it's different depending on who you want to live the most. The title for the story would be Hear Me, as they just want to prove their existence to each other.
Is that a good enough idea for you?