So, for my answers I wanted to divide my answer into two different sections. What is game Design and How do you implement game design? The reasons that I felt the needed to separate the two were because they kind of go hand in hand in a sense.I feel like depending on the design of the game you need to approach the development process differently. For example for my game, which is a Visual Novel that is heavily choice driven. I originally started by going and doing Set Design, but I quickly realized that I could not really do that as a lot of the story had to be based off of what was happening in the current scene. Meaning I had to go back and change sets, Reimage characters, etc. So, instead I had to go back and take that to a base level and really take my time with a firm and polished story setup. With that being said, game design is a creative outline on which you want your game to function and that is based off of whether you want it to be competitive, adventurous, Open-Word, Third-Person, Etc. I feel as if it is the thing that will determine what gamemodes your game will have, how many people can play together, whether you can die or not, how the graphics look as well as more. The Design of your game is the "Image" of your game it is the first impression that people will get when fully playing. For example, i will use the Doom games. They have a very basic design. You run through Semi-Linear levels and blow up and kill stuff. And that basic concept is super fine and dandy but it mixed ion other Design aspects such as the iconic Doom Sound track that is really catchy and fun, the blood and gore that comes with doing your objective, as well as the innovative first person perspective that it gave players at the time.
Now, as far as implementing the game's design is key as well because the way you implement your tools is almost like be the lead for an orchestra. You steer people's emotions in the way that you want them to go. A ship can't be lead in a straight path unless the helms man keeps it on course and you as the captain need to make that happen. Without that your game could begin to lose it's structure and have players confused on what is happening. It will also tend to lose people's attention as well as leave the game unmarketable because it doesn't have those "wow" or "woah" moments. I'll use Doom as another example for this. The music of Doom varies due to what section of the game you are in. How many enemies' are on the map, "are you discovering something?", is there a Boss enemy spawning?, Etc.