Anyone can make an FPS out of the Unreal Engine, it doesn't mean they'll be sued for it because they're using Epic Game's engine. Same with us, we can make a gameplay mechanic mimic another title, but it doesn't necessarily have to be using the same textures, designs, or heck, even need to be blocky (though if we did go through with it, it would be blocky to retain the feel of the original in some sense).
Essentially, there have been numerous titles out there to try and better render another game (take a look at the App Store when you search up Pixel Gun 3D, and tell me how many people tried to mimic them?) Besides, as I'm saying, we would be using the mechanics of the gameplay, which you can't exactly copyright, because then every single pixelated FPS would be removed (even those before Pixel Gun 3D). If we used the textures directly from the game, we would be under fire, but if it used completely different designs, it would become a Pixel Gun 3D "clone" essentially (which is what half the pixelated FPS games on the App Store are today).