When I heard 'Only one' I immediately went to 'THERE CAN ONLY BE ONE' and the elimination aspect of it, going from having a lot being whittled down until there is only one(or almost only one anyway). Thinking of it like a single player battle royal or a musical chairs like game where you start up controlling a load of characters, until the environment reduces it down to potentially only one as you sacrifice multiple character to solve puzzles to save that one.
From that I swapped musical chairs with zombies that would slowly assimilate your crowd, and while I deiced not to be a stickler of forcing the player to have to reduce there crowd to one person to pass the level, they only needed one person to survive to win. Apart from that, I just tried to keep everything minimalist, colors are mostly all green, all the charters act as one and the like.
My game a quick to play zombie survival puzzle game; https://itch.io/jam/gmtk-2019/rate/461074