Direct quote 1997
“In game design, I think the theme of a game occupies the same position of importance as the gameplay. The hardest part for me of any development is building a theme that will enliven and enrich the gameplay.”
What it documents
At a Game Hihyou roundtable while working on Metal Gear Solid, Kojima said that in game design the theme of a game occupies the same position of importance as the gameplay, and that the hardest part for him of any development is building a theme that will enliven and enrich the gameplay; he defined game design as the practical work of reconciling and combining those two elements—what kind of story is going to make the gameplay come alive and what kind of gameplay is necessary to advance the story.
Source
shmuplations.com; "What is Game Design?" roundtable in Game Hihyou magazine, unofficial English translation, 1997 (Kojima's contributions). View source ↗
“What kind of story is going to make the gameplay come alive? Likewise, what kind of gameplay is necessary to advance the story? I think "game design" is the practical work of reconciling and combining these two elements in a game.”