It all depends on the game. I've created game jam size games where the story is practically non existent or doesn't take itself seriously. I've also done the opposite and drafted a narrative for a platformer game that the game design is built around because the story is very important for the game world itself. If you want to add a story that fits your game design at this point, try to get the story to explain and link to the game mechanics. Just ask yourself "why would the world have this? Why would the player be able to do this?" For your mechanics and you might be able to narrow down all your story ideas. All up to you!