Well, there are several methods on coming up with game designs.
Method 1 - Exploration
Let’s say you open studio one day and you created a car, and this thought hits you “What will I do with this car now?” and this is basically where game ideas come in. Possibly you may decide to create a game with characters as cars similar to the movie Cars by Pixar with a similar story line or create some sort of car derby game with destructive weapons and upgrades or even a racing game. This is just the basics of coming up with a game design. From here, ideas are bound to come in, especially if they make sense to your development style.
Method 2 - Inspiration
If you still can’t come up with a game design, take a walk in the part, listen to music, play other games, watch movies, search on Pinterest or Instagram etc. You will surely find great sources of inspiration from these places and will lead to a strong game design, however one challenge is keeping up with development fatigue. Inspiration dies out fairly easily, possibly due to personal issues, family issues, time management, poor development team etc. When this strikes you, it is possble that you may want to start on a new project, no, don’t let that fatigue eat you up. As soon as possible, create a concept of your game in studio, possibly a photo or video and whenever you have fatigue, look back on these concepts, it will push you to continue 
(There are some posts on the forum about Development Fatigue, you may want to look into those)
Method 3 - Randomness
Fairly similar to Method 1, however this is like sorta-copying an existing game, but with twists. If you want to create a game like Mad City where criminals could have hero powers right from the spawn or team selection or even a racing system, that’s a fine thing to do, however don’t directly copy stuff from Mad City because this could lead to some serious issues. As much as possible, create original assets. Indirectly creating a sorta-similar game to other existing game may be tricky, especially if you ask yourself “Is that it?” usually existing games are already as complete as they are, which is why you need to make you choice of selection very carefully.
I’ve answered questions like yours several times, you may check them out if you want, but as time goes on, I improve my replies. I hope this helps you find a solid game idea
Here are some if not all of them: