If your game allows people to hold stuff: A small purple Stegosaurus that the player can pick up. It ever so slightly grows in size whenever you enter a new room. Once it is big enough you can use its tail as a ramp to get to a new area, otherwise inaccessible.
The issue is not the genre “single player (shooter)” itself, but that these big companies just churn out the same generic bullshit and then act surprised when no-one plays it.
AAA studios just don’t have the balls anymore to take a risk and develop something unique. And this is their downfall.
Titanfall 2, Metro Exodus, Ghostwire Tokyo, Doom (to name a few) are all excellent first person shooters. All of them have something unique about them that makes them worthwhile.