Quite often game design isn’t about creating your idea, but you might need to take an existing mechanics and make a great game from it.
In a way game designer needs to be able to create interesting games starting from just the mechanics, or story or theme and then to add the rest!
Here’s an example.
Did asteroids start like this? Just the mechanics.
It ended up being space themed.
It could have been about saving the Olympics from rain and you flying that plane breaking the clouds :D
Most Tetris games you see are the same. Colorful blocks without any actual theme.
But what could the theme be? There isn't many examples available, but here's one: